feat(snapshot): re-encode captures + disk-pressure retention

Camera snapshots were stored RAW — the camera's full-res JPEG straight into
the BLOB, no resize/recompress. Measured on the dev DB: 300 snapshots = 81.7 MB
= ~72% of the 114 MB SQLite file (the big ones 2688×1520 / ~600 KB, Hikvision
main stream). They dominated the appliance's single backed-up DB file.

Re-encode on capture (snapshot.ts):
- Downscale each frame to SNAPSHOT_MAX_EDGE (1280px long edge) + recompress at
  SNAPSHOT_JPEG_QUALITY (80) via sharp (libvips, Apache-2.0) before storage —
  ~6-10× smaller (verified 2688×1520 → 1280×724, ~8×), plate still readable,
  clean image/jpeg (drops the camera's charset cruft). STORAGE-ONLY: recognition
  keeps the ORIGINAL full-res bytes (downscaling hurts OCR). Fail-soft — a
  re-encode error stores the original, never drops the snapshot or blocks the
  (already-open) path. sharp lives in apps/server (owns the capture path), where
  bcrypt already establishes the native-dep pattern.

Disk-pressure retention (snapshot-retention.ts) — a SAFETY VALVE, not the daily
mechanism (the re-encode does that). Daily check reads the DB filesystem used%
(statfs on db.$client.name); no-op unless ≥ SNAPSHOT_DISK_HIGH_PCT (70). Over the
mark: delete the OLDEST until an estimated SNAPSHOT_DISK_FREE_TARGET_PCT (10%) of
disk is freed — never below SNAPSHOT_MIN_KEEP (500) — then VACUUM once to return
space to the OS. A DELETE only frees SQLite pages (disk doesn't drop until VACUUM),
so the loop is driven by estimated freed bytes (SUM(length(bytes))), not a live
disk re-read; the prune owns the DB-locking VACUUM, run daily off-peak. diskUsage
is injectable for tests. None of this touches the signed ledger — snapshots are
unsigned/advisory, referenced only by id.

Tests: encodeForStorage (downscale / clean-type / no-enlarge / fail-soft) +
pruneSnapshots (no-op below mark / delete-oldest-to-target + VACUUM / MIN_KEEP
floor / skip-VACUUM-when-empty). All four snapshot env knobs documented in the
komodo env reference. Full workspace build/lint/test green; the prune smoke-verified
on a scratch DB copy (file shrank after VACUUM).

Existing ~81.7 MB of raw snapshots are unchanged (a one-off re-encode backfill is
a separate optional follow-up). Updated entry-exit-points + technology-stack wiki.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/server/src/snapshot-retention.test.ts

@@ -0,0 +1,96 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { snapshots, type Db } from "@parking/db";
+import { createTestDb } from "@parking/db/testing";
+import { pruneSnapshots, type DiskUsage, type SnapshotRetention } from "./snapshot-retention.js";
+
+// Snapshot retention: DISK-PRESSURE prune. No-op unless the DB's filesystem is over the
+// high-water mark; then delete the OLDEST until ~freeTargetPct of disk is freed (estimated from
+// the deleted BLOB sizes), honoring a MIN_KEEP floor, then VACUUM once. Disk usage is injected
+// so the test controls the trigger without touching the real filesystem.
+
+let db: Db;
+beforeEach(() => {
+  ({ db } = createTestDb());
+});
+
+/** Insert `n` snapshots, oldest first (s-0 is the oldest), each `bytes` long. */
+function seed(n: number, bytes = 1000): void {
+  const t0 = Date.now() - n * 1000;
+  for (let i = 0; i < n; i++) {
+    db.insert(snapshots)
+      .values({
+        id: `s-${i}`,
+        direction: "entry",
+        deviceId: "cam",
+        identity: `s-${i}`,
+        contentType: "image/jpeg",
+        bytes: Buffer.alloc(bytes, 1),
+        capturedAt: new Date(t0 + i * 1000).toISOString(), // s-0 oldest … s-(n-1) newest
+      })
+      .run();
+  }
+}
+
+function count(): number {
+  return db.select().from(snapshots).all().length;
+}
+function ids(): string[] {
+  return db.select().from(snapshots).all().map((r) => r.id).sort();
+}
+
+/** A fake disk at a given used% on a 1 GB volume. */
+const disk = (usedPct: number, totalBytes = 1_000_000_000): (() => Promise<DiskUsage>) =>
+  () => Promise.resolve({ usedPct, totalBytes });
+
+const ret = (o: Partial<SnapshotRetention>): SnapshotRetention => ({
+  highPct: 70,
+  freeTargetPct: 10,
+  minKeep: 2,
+  batch: 5,
+  ...o,
+});
+
+describe("pruneSnapshots (disk-pressure)", () => {
+  it("no-op when disk is below the high-water mark", async () => {
+    seed(10);
+    const res = await pruneSnapshots(db, { retention: ret({}), diskUsage: disk(50) });
+    expect(res.deletedRows).toBe(0);
+    expect(res.vacuumed).toBe(false);
+    expect(count()).toBe(10);
+  });
+
+  it("over the mark: deletes the OLDEST until ~freeTargetPct is freed, then VACUUMs", async () => {
+    // 1 GB disk, target 10% = 100 MB. Each snapshot 20 MB → ~5 deletions reach the target.
+    seed(20, 20 * 1048576);
+    const vacuumSpy = vi.spyOn(db.$client as { exec: (s: string) => void }, "exec");
+    const res = await pruneSnapshots(db, { retention: ret({ minKeep: 2, batch: 100 }), diskUsage: disk(80) });
+    expect(res.deletedRows).toBeGreaterThanOrEqual(5);
+    expect(res.freedBytesEst).toBeGreaterThanOrEqual(0.1 * 1_000_000_000);
+    expect(res.vacuumed).toBe(true);
+    expect(vacuumSpy).toHaveBeenCalledWith("VACUUM");
+    // The survivors are the NEWEST (oldest went first).
+    const survivors = ids();
+    expect(survivors).toContain(`s-19`); // newest kept
+    expect(survivors).not.toContain(`s-0`); // oldest pruned
+    vacuumSpy.mockRestore();
+  });
+
+  it("honors the MIN_KEEP floor even when still over target", async () => {
+    // Target 10% of 1 GB = 100 MB, but only 3 tiny snapshots exist and minKeep=2 → at most 1 deleted.
+    seed(3, 1000);
+    const res = await pruneSnapshots(db, { retention: ret({ minKeep: 2, freeTargetPct: 90 }), diskUsage: disk(95) });
+    expect(res.deletedRows).toBe(1); // 3 − minKeep(2)
+    expect(count()).toBe(2);
+    expect(res.floorHitWhileOver).toBe(true); // couldn't reach target without crossing the floor
+  });
+
+  it("skips VACUUM when nothing was deleted", async () => {
+    seed(2); // == minKeep, so nothing to delete even over the mark
+    const vacuumSpy = vi.spyOn(db.$client as { exec: (s: string) => void }, "exec");
+    const res = await pruneSnapshots(db, { retention: ret({ minKeep: 2 }), diskUsage: disk(99) });
+    expect(res.deletedRows).toBe(0);
+    expect(res.vacuumed).toBe(false);
+    expect(vacuumSpy).not.toHaveBeenCalled();
+    vacuumSpy.mockRestore();
+  });
+});