server+web: capacity / FULL gate (occupancy fold + transient refuse)

Occupancy is a fold over the signed ledger (entries minus exits per identity);
getOccupancy returns {count, capacity, free, full}. Capacity is a single-row
site_config table (admin-set; null = uncapped; migration 0001, additive).

FULL gate lives in the transient entry flow: when full, refuse (no ticket, no
vehicle_entry, no open) and sign an anomaly. Permit entry is NOT gated --
subscribers are admitted past transient-full (their own maxConcurrent still
applies), so occupancy can read over capacity by design (reserve-for-permits).

Routes: GET /api/occupancy + GET /api/site-config (any role), PUT
/api/site-config (admin; non-negative int or null). Web SiteSettings: live
occupancy + FULL badge (everyone), capacity editor (admin).

Verified: fill to cap -> 3rd transient refused; permit admitted past full; exit
frees a slot; RBAC (operator can't set, -5 -> 400); verifyChain ok. Physical
FULL-sign relay output deferred.

apps/server/src/entry-flow.ts

@@ -11,6 +11,7 @@ import {
 } from "@parking/devices";
 import type { FastifyBaseLogger } from "fastify";
 import type { DeviceInputEvent } from "./device-events.js";
+import { getOccupancy } from "./occupancy.js";
 import type { EventLog } from "./event-log.js";
 import type { LaneMap } from "./lane-map.js";
 
@@ -77,6 +78,22 @@ export class EntryFlow {
   }
 
   async #runEntry(lane: number, input: number, access: AccessControlDevice): Promise<void> {
+    // CAPACITY GATE (transient only). When the lot is full, refuse transient entry:
+    // no ticket, no vehicle_entry, no open — sign an anomaly. Permit holders are NOT
+    // gated here (their flow ignores site-full; their own maxConcurrent applies), so
+    // subscribers aren't locked out. "Full" is a soft policy seam for valet over-
+    // capacity later. See wiki/concepts/capacity-occupancy.md.
+    const occ = getOccupancy(this.#db);
+    if (occ.full) {
+      await this.#log.append({
+        type: "anomaly",
+        lane,
+        payload: { reason: `transient entry refused — lot full (${occ.count}/${occ.capacity})`, entryRefused: true, full: true },
+      });
+      this.#logger.warn(`transient entry REFUSED on lane ${lane}: full (${occ.count}/${occ.capacity})`);
+      return;
+    }
+
     const ticketId = newTicketId();
     const issuedAt = new Date().toISOString();
     const printers = await this.#loadPrinters(lane);

apps/server/src/occupancy.ts

@@ -0,0 +1,49 @@
+import { eq, ledgerEvents, siteConfig, type Db } from "@parking/db";
+
+// Occupancy = a FOLD over the signed ledger: the count of vehicle_entry events
+// with no matching vehicle_exit. Never a hand-maintained counter (which is
+// editable + drifts) — the chain is the truth. See wiki/concepts/capacity-occupancy.md.
+
+export interface Occupancy {
+  /** Cars currently inside (open sessions). */
+  readonly count: number;
+  /** Admin-set nominal capacity, or null = no limit. */
+  readonly capacity: number | null;
+  /** capacity − count, or null when uncapped. Can read 0 (or below) when full. */
+  readonly free: number | null;
+  /** True when count ≥ capacity (always false when uncapped). */
+  readonly full: boolean;
+}
+
+/** Count cars inside: entries minus exits, per identity, over the ledger. */
+export function occupancyCount(db: Db): number {
+  const rows = db
+    .select({ type: ledgerEvents.type, identity: ledgerEvents.identity })
+    .from(ledgerEvents)
+    .all();
+  const balance = new Map<string, number>();
+  for (const r of rows) {
+    if (r.type === "vehicle_entry") balance.set(r.identity ?? "", (balance.get(r.identity ?? "") ?? 0) + 1);
+    else if (r.type === "vehicle_exit") balance.set(r.identity ?? "", (balance.get(r.identity ?? "") ?? 0) - 1);
+  }
+  let open = 0;
+  for (const v of balance.values()) if (v > 0) open += 1;
+  return open;
+}
+
+/** Admin-set capacity (null = uncapped). */
+export function siteCapacity(db: Db): number | null {
+  const row = db.select().from(siteConfig).where(eq(siteConfig.id, 1)).get();
+  return row?.capacity ?? null;
+}
+
+export function getOccupancy(db: Db): Occupancy {
+  const count = occupancyCount(db);
+  const capacity = siteCapacity(db);
+  return {
+    count,
+    capacity,
+    free: capacity == null ? null : capacity - count,
+    full: capacity != null && count >= capacity,
+  };
+}

apps/server/src/routes/site.ts

@@ -0,0 +1,43 @@
+import type { FastifyInstance } from "fastify";
+import { eq, siteConfig, type Db } from "@parking/db";
+import { requireRole } from "../auth.js";
+import { getOccupancy } from "../occupancy.js";
+
+// Site config (capacity) + live occupancy. Occupancy is a fold over the signed
+// ledger; capacity is an admin-set knob. The FULL gate (refuse transient entry at
+// capacity) lives in the entry flow. See wiki/concepts/capacity-occupancy.md.
+
+interface SiteConfigBody {
+  /** Nominal capacity; null = no limit. */
+  capacity: number | null;
+}
+
+export async function siteRoutes(app: FastifyInstance, db: Db): Promise<void> {
+  const readGuard = requireRole("admin", "operator", "cashier", "readonly");
+  const writeGuard = requireRole("admin");
+
+  // Live occupancy: cars inside, capacity, free, full. Any signed-in role.
+  app.get("/api/occupancy", { preHandler: readGuard }, async () => getOccupancy(db));
+
+  // Read site config (capacity).
+  app.get("/api/site-config", { preHandler: readGuard }, async () => {
+    const row = db.select().from(siteConfig).where(eq(siteConfig.id, 1)).get();
+    return { capacity: row?.capacity ?? null };
+  });
+
+  // Set capacity (admin). null or 0+ integer.
+  app.put<{ Body: SiteConfigBody }>("/api/site-config", { preHandler: writeGuard }, async (req, reply) => {
+    const { capacity } = req.body ?? ({} as SiteConfigBody);
+    if (capacity != null && (!Number.isInteger(capacity) || capacity < 0)) {
+      return reply.code(400).send({ error: "capacity must be a non-negative integer or null" });
+    }
+    const existing = db.select().from(siteConfig).where(eq(siteConfig.id, 1)).get();
+    const updatedAt = new Date().toISOString();
+    if (existing) {
+      db.update(siteConfig).set({ capacity: capacity ?? null, updatedAt }).where(eq(siteConfig.id, 1)).run();
+    } else {
+      db.insert(siteConfig).values({ id: 1, capacity: capacity ?? null, updatedAt }).run();
+    }
+    return { capacity: capacity ?? null };
+  });
+}

apps/server/src/server.ts

@@ -21,6 +21,7 @@ import { eventRoutes } from "./routes/events.js";
 import { payRoutes } from "./routes/pay.js";
 import { permitRoutes } from "./routes/permits.js";
 import { shiftRoutes } from "./routes/shift.js";
+import { siteRoutes } from "./routes/site.js";
 import { tariffRoutes } from "./routes/tariffs.js";
 import { printerRoutes } from "./routes/printers.js";
 import { setupRoutes } from "./routes/setup.js";
@@ -129,6 +130,10 @@ export async function buildServer(opts: BuildOptions = {}): Promise<FastifyInsta
   const shiftService = new ShiftService(db, eventLog, app.log);
   await shiftRoutes(app, shiftService);
 
+  // Site config (capacity) + live occupancy. The FULL gate (refuse transient entry
+  // at capacity) is in the entry flow. See wiki/concepts/capacity-occupancy.md.
+  await siteRoutes(app, db);
+
   const unsubscribeInput = deviceEvents.onInput((e) => {
     // Resolve which lane the device belongs to. -1 marks "device fired but isn't
     // mapped to a lane" (assigned without a lane, or a stale id) — still recorded