feat(scanner): quiet scans, enqueue coalescing, watcher probe, sibling cache

Part 2 of the faceted plan, Phase 5 (steps 5-6): the scan that used to
print ~4 lines per unchanged file now prints ONE summary line, and a
poll tick landing mid-scan no longer stacks a second full scan.

- Quiet scans (§4): the walk loop is silent — inserts, changes, and
  errors log inside processMediaFile; the duplicate
  'Successfully processed' branch (both branches printed it) and the
  per-file 'processing / already exists / same size, skipping' chatter
  are gone. Summary carries the trigger and a changed counter:
  'Scan of <folders> (trigger: poll): N files, N changed, N new, Xs,
  N errors'. Triggers threaded through every enqueue site: poll
  (startBackupScan), watcher (dirty-dirs flush), admin (scan API),
  initial (startup scan).
- Enqueue coalescing (§5): Worker.HasActiveScanForLibrary (JobResult
  gains additive job_type so only SCAN jobs coalesce — an import
  sharing the library id never blocks one). enqueueLibraryScan skips
  silently; the admin scan API answers 409 with the active job id.
  Enforces the one-scan-per-library invariant the bulk scan-state map
  leans on.
- Watcher capability probe (§4): ONE Add on a root folder; failure
  disables the watcher with a single startup line (replaces the
  per-directory warning spam on inotify-broken deployments). The
  watcher stays an optional latency reducer over scheduled scans.
- Per-directory sibling cache (§6): hasSiblingBookFileCached caches
  the ReadDir per directory for one scan — loose-image manga re-stats
  thousands of images sharing a handful of directories.

Tests: TestHasActiveScanForLibrary (unit — running/pending coalesce,
terminal jobs don't, non-scan jobs don't, empty library never, nil
worker safe). Full scanner suite green in the container.

docs/contract_pins.md

@@ -219,3 +219,21 @@ cover_image_path, has_conflict, format_group, badge`.
   re-extraction (deploy-day rule). Multifile folder/zip books keep the
   Phase 3 behavior (their change path does not consume sidecar
   extractors yet — gating them would flip books "changed" for nothing).
+
+## 2026-10-19 — Scanner Phase 5: quiet scans, coalescing, watcher probe
+
+- `POST /api/scanner/scan` now answers **409** (with `active_job_id`)
+  when a scan for the same library is already pending/running —
+  coalescing instead of stacking a second full scan. The endpoint was
+  never pinned; no existing response shape changed. Poll ticks and
+  watcher events coalesce silently (no second job enqueued).
+- Scan jobs carry a `trigger` label (poll/watcher/admin/initial); the
+  per-scan log is now ONE summary line:
+  `Scan of <folders> (trigger: <t>): N files, N changed, N new, Xs, N errors`.
+  Per-file "processing/success/skipping" chatter is gone; inserts,
+  changes, and errors still log individually.
+- `JobResult` gains additive `job_type` (coalescing needs it; the
+  scan-progress surfaces tolerate unknown keys).
+- Watcher: a failed capability probe (one `Add` on a root folder)
+  disables the watcher with ONE startup line instead of per-directory
+  warning spam.