feat(server): live booth WebSocket feed (/api/ws)

Add @fastify/websocket. EventLog fires an onAppended callback after each durable
append; device-events gains a ledger channel (emitLedger). /api/ws fans out
ledger + occupancy + printer-status to authenticated booth clients. Origin
allowlist (WS_ALLOWED_ORIGINS) replaces CSRF for the handshake (anti-CSWSH).

Note: server.ts also reflects later booth route wiring; the final HEAD builds.

apps/server/src/device-events.ts

@@ -1,5 +1,6 @@
 import { EventEmitter } from "node:events";
 import type { PrinterStatus } from "@parking/devices";
+import type { LedgerEventRow } from "@parking/db";
 
 // Internal event bus for device-originated events (button presses, etc.).
 // Hardware drivers / inbound device pushes emit here; business logic (entry
@@ -74,6 +75,21 @@ class DeviceEventBus extends EventEmitter {
     this.on("printer-status", cb);
     return () => this.off("printer-status", cb);
   }
+
+  /**
+   * Emitted AFTER a signed business event is appended to the ledger (entry, exit,
+   * payment, void, …). The payload is the persisted row — business facts only, no
+   * secrets — so it is safe to fan out to authenticated booth clients over the WS.
+   * This is a read-side notification ONLY: it never feeds back into append/sign/
+   * chain logic. See event-log.ts (emitted from EventLog.append) and routes/ws.ts.
+   */
+  emitLedger(event: LedgerEventRow): void {
+    this.emit("ledger", event);
+  }
+  onLedger(cb: (event: LedgerEventRow) => void): () => void {
+    this.on("ledger", cb);
+    return () => this.off("ledger", cb);
+  }
 }
 
 /** Process-wide device event bus. */

apps/server/src/event-log.ts

@@ -92,13 +92,23 @@ export class EventLog {
    *  (JWT-fallback → dedicated key → ATECC608). Defaults to the append signer for
    *  callers that don't pass one (single-key chains, tests). */
   readonly #resolveVerifier: SignerResolver;
+  /** Optional read-side notification, fired AFTER a row is durably inserted. Used
+   *  to fan the event out to live booth clients (WS). It is best-effort and must
+   *  NOT influence the append/sign/chain path — a throwing/absent sink is ignored. */
+  readonly #onAppended?: (row: LedgerEventRow) => void;
   /** Serialize appends: each waits for the previous to finish. */
   #tail: Promise<unknown> = Promise.resolve();
 
-  constructor(db: Db, signer: Signer, resolveVerifier?: SignerResolver) {
+  constructor(
+    db: Db,
+    signer: Signer,
+    resolveVerifier?: SignerResolver,
+    onAppended?: (row: LedgerEventRow) => void,
+  ) {
     this.#db = db;
     this.#signer = signer;
     this.#resolveVerifier = resolveVerifier ?? (() => signer);
+    this.#onAppended = onAppended;
   }
 
   /** Append one event to the chain. Returns the persisted row. Serialized. */
@@ -106,7 +116,16 @@ export class EventLog {
     const run = this.#tail.then(() => this.#appendNow(input));
     // Keep the chain going even if one append rejects (don't wedge the lock).
     this.#tail = run.catch(() => undefined);
-    return run;
+    // Read-side notification, AFTER the row is durably written. Wrapped so a
+    // failing sink can never reject the append or break the chain lock above.
+    return run.then((row) => {
+      try {
+        this.#onAppended?.(row);
+      } catch {
+        // best-effort fan-out only — swallow.
+      }
+      return row;
+    });
   }
 
   #appendNow(input: AppendInput): LedgerEventRow {

apps/server/src/routes/ws.ts

@@ -0,0 +1,104 @@
+import type { FastifyInstance } from "fastify";
+import type { Db } from "@parking/db";
+import type { Role } from "@parking/shared";
+import { deviceEvents } from "../device-events.js";
+import { getOccupancy } from "../occupancy.js";
+
+// Live booth feed over a WebSocket. The booth UI opens ONE socket and receives
+// server-pushed updates instead of polling: each signed ledger append (entry,
+// exit, payment, void) is fanned out, and the recomputed occupancy rides along
+// so the screen's count stays exact (occupancy is a fold over the same ledger,
+// never a counter). Printer-status changes are forwarded too.
+//
+// Auth: the handshake is a normal GET through Fastify's lifecycle, so the same
+// HttpOnly JWT cookie that guards the REST API guards this. We verify the JWT and
+// role here. A browser's WebSocket constructor cannot set custom headers, so the
+// CSRF double-submit header the REST mutations use is unavailable — which would
+// leave the socket open to Cross-Site WebSocket Hijacking: a malicious page in the
+// operator's browser could open ws://<booth>/api/ws, the browser would auto-attach
+// the HttpOnly cookie, and the attacker would receive the live entry/exit/payment
+// stream. The cookie alone is NOT a control here. So we replace the CSRF check with
+// an Origin allowlist: the handshake's Origin must be same-origin (or an explicitly
+// allowed booth UI origin). Non-browser clients (no Origin) are rejected too.
+// See auth.ts, event-log.ts (emitLedger), capacity-occupancy.md.
+
+/** Roles allowed to watch the live feed (everyone signed in; readonly included —
+ *  it's a read-only stream). */
+const WATCH_ROLES: Role[] = ["admin", "operator", "cashier", "readonly"];
+
+/**
+ * Is the handshake's Origin trusted? Same-origin (Origin host === Host header) is
+ * always allowed; additional origins can be allowlisted via WS_ALLOWED_ORIGINS
+ * (comma-separated) for a booth UI served from a different origin. A missing or
+ * mismatched Origin is rejected — that is the anti-CSWSH control.
+ */
+function isAllowedOrigin(origin: string | undefined, host: string | undefined): boolean {
+  if (!origin) return false; // no Origin → not a same-origin browser request
+  let originHost: string;
+  try {
+    originHost = new URL(origin).host;
+  } catch {
+    return false; // malformed Origin
+  }
+  if (host && originHost === host) return true; // same-origin (any scheme/port match via host)
+  const allow = (process.env.WS_ALLOWED_ORIGINS ?? "")
+    .split(",")
+    .map((s) => s.trim())
+    .filter(Boolean);
+  return allow.includes(origin);
+}
+
+type OutMsg =
+  | { kind: "hello"; occupancy: ReturnType<typeof getOccupancy> }
+  | { kind: "ledger"; event: unknown; occupancy: ReturnType<typeof getOccupancy> }
+  | { kind: "printer-status"; event: unknown };
+
+export async function wsRoutes(app: FastifyInstance, db: Db): Promise<void> {
+  app.get(
+    "/api/ws",
+    {
+      websocket: true,
+      // Origin allowlist (anti-CSWSH, replaces CSRF — see file header) THEN JWT +
+      // role. Reject a cross/absent origin before touching the token, so a hijack
+      // attempt never reaches an authenticated socket. jwtVerify reads the cookie.
+      preHandler: async (req) => {
+        if (!isAllowedOrigin(req.headers.origin, req.headers.host)) {
+          throw Object.assign(new Error("forbidden origin"), { statusCode: 403 });
+        }
+        await req.jwtVerify();
+        if (!req.user || !WATCH_ROLES.includes(req.user.role)) {
+          throw Object.assign(new Error("forbidden"), { statusCode: 403 });
+        }
+      },
+    },
+    (socket) => {
+      const send = (msg: OutMsg) => {
+        // readyState 1 = OPEN; never throw out of an event-bus callback.
+        if (socket.readyState === 1) {
+          try {
+            socket.send(JSON.stringify(msg));
+          } catch {
+            /* drop on a broken socket */
+          }
+        }
+      };
+
+      // Initial snapshot so the client renders immediately, before any event.
+      send({ kind: "hello", occupancy: getOccupancy(db) });
+
+      // Subscribe to the live buses. Each handler recomputes occupancy from the
+      // ledger (cheap fold) so the pushed count is always authoritative.
+      const offLedger = deviceEvents.onLedger((event) => {
+        send({ kind: "ledger", event, occupancy: getOccupancy(db) });
+      });
+      const offPrinter = deviceEvents.onPrinterStatus((event) => {
+        send({ kind: "printer-status", event });
+      });
+
+      socket.on("close", () => {
+        offLedger();
+        offPrinter();
+      });
+    },
+  );
+}

apps/server/src/server.ts

@@ -1,5 +1,6 @@
 import cookie from "@fastify/cookie";
 import jwt from "@fastify/jwt";
+import websocket from "@fastify/websocket";
 import Fastify, { type FastifyInstance } from "fastify";
 import { randomUUID } from "node:crypto";
 import { createDb, deviceEvents as deviceEventsTable, type Db } from "@parking/db";
@@ -26,6 +27,7 @@ import { snapshotRoutes } from "./routes/snapshots.js";
 import { tariffRoutes } from "./routes/tariffs.js";
 import { printerRoutes } from "./routes/printers.js";
 import { setupRoutes } from "./routes/setup.js";
+import { wsRoutes } from "./routes/ws.js";
 
 // The backend is Fastify (Node). Hardware drivers live as isolated Fastify
 // plugins emitting onto a shared internal event bus; auth is fully local
@@ -44,6 +46,10 @@ export async function buildServer(opts: BuildOptions = {}): Promise<FastifyInsta
 
   await app.register(cookie);
 
+  // WebSocket support for the live booth feed (/api/ws). Registered before the
+  // routes so the `{ websocket: true }` route option is available.
+  await app.register(websocket);
+
   // Local JWT signing with a local secret — no external identity provider.
   // Fail fast rather than fall back to a known default: a booth machine started
   // without a real secret would sign tokens anyone could forge (incl. an admin
@@ -86,9 +92,17 @@ export async function buildServer(opts: BuildOptions = {}): Promise<FastifyInsta
   // in device_events. The entry flow (TODO) turns an input into a signed
   // vehicle_entry once a ticket prints + the barrier is commanded.
   // See wiki/decisions/event-streams-split.md.
-  const eventLog = new EventLog(db, buildSigner(app.log), buildVerifier);
+  // The 4th arg is a read-side fan-out fired AFTER each durable append — used to
+  // push the event to live booth clients (WS). It cannot affect the sign/chain path.
+  const eventLog = new EventLog(db, buildSigner(app.log), buildVerifier, (row) =>
+    deviceEvents.emitLedger(row),
+  );
   await eventRoutes(app, db, eventLog);
 
+  // Live booth feed: server-pushed ledger + occupancy + printer-status over a
+  // single authenticated WebSocket (/api/ws). See routes/ws.ts.
+  await wsRoutes(app, db);
+
   // Entry/exit camera snapshots (BLOB-in-DB), read-only. See snapshot.ts.
   await snapshotRoutes(app, db);
 
@@ -121,7 +135,7 @@ export async function buildServer(opts: BuildOptions = {}): Promise<FastifyInsta
   // Pay station (pay-on-foot): quote an open session against the active tariff +
   // take payment → signed `payment` event. See wiki/concepts/tariff.md.
   const payStation = new PayStation(db, eventLog, app.log);
-  await payRoutes(app, payStation);
+  await payRoutes(app, db, payStation, exitFlow);
 
   // Tariff composer: admin publishes effective-dated, immutable rate-card versions
   // the pay station prices against. See wiki/concepts/tariff.md.