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.

internal/handlers/scanner.go

@@ -84,6 +84,23 @@ func (h *Handler) ScanLibrary(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "either library_id or folder_paths required for scanning"})
 	}
 
+	// Coalescing (plan Part 2 §5): a scan for this library is already
+	// pending or running — 409 with the active job so the UI can show
+	// progress instead of stacking a second full scan.
+	if req.LibraryID != "" && h.worker.HasActiveScanForLibrary(req.LibraryID) {
+		var activeJobID string
+		for _, active := range h.worker.GetActiveScanJobs() {
+			if active.JobType == services.JobTypeScan && active.LibraryID == req.LibraryID {
+				activeJobID = active.JobID
+				break
+			}
+		}
+		return c.JSON(http.StatusConflict, map[string]string{
+			"error":         "a scan for this library is already pending or running",
+			"active_job_id": activeJobID,
+		})
+	}
+
 	jobID := uuid.New().String()
 
 	job := &services.Job{
@@ -96,6 +113,7 @@ func (h *Handler) ScanLibrary(c *echo.Context) error {
 			"admin_id":   userUUID.String(),
 			"db":         h.db,
 			"force":      req.Force,
+			"trigger":    "admin",
 		},
 		Status:  services.JobStatusPending,
 		Context: h.ctx,