feat(koreader): device bridge for ratings, reviews and reading-time stats

Extends the device-authed koreader surface with the handoff item 4
Batch B contract:

- GET/POST /api/sync/koreader/rating — the device mirrors the .sdr
  summary's 5-star rating against the 1-10 server scale (POST 0 clears).
- GET/POST /api/sync/koreader/review — raw-markdown review mirror
  (summary.note); empty body clears, matching the first-party PUT.
- POST /api/sync/koreader/stats — the device pushes its CUMULATIVE
  total reading seconds; the server watermark (highest total ever
  converted, read from the koreader sessions' own device_metadata)
  converts only the delta into a reading_history session attributed
  to the koreader device. Reinstalls cannot double-count; sub-minute
  deltas ride forward (no session, no watermark movement); a lagging
  device clamps to zero instead of dragging the total back.

Book identity resolves through the existing priority chain (uuid →
sha256 → alias → title/author). GET binding uses query tags; POST
endpoints bind the ref+payload once (an earlier resolve-then-bind
draft drained the body).

Live-verified against the dev server with the koreader device token:
rating set/get/clear, review mirror, and the stats sequence
900→session, 960→delta session, 970→rides forward, 500→clamps; the
book's stats card and the By Device panel both pick up the split.

internal/database/querier.go

@@ -235,6 +235,12 @@ type Querier interface {
 	GetKoboShelfBooksByShelfName(ctx context.Context, arg GetKoboShelfBooksByShelfNameParams) ([]GetKoboShelfBooksByShelfNameRow, error)
 	// Get Kobo shelves by collection
 	GetKoboShelvesByCollection(ctx context.Context, arg GetKoboShelvesByCollectionParams) ([]GetKoboShelvesByCollectionRow, error)
+	// KOReader stats conversion: the device pushes its CUMULATIVE total reading
+	// seconds for a book; the watermark is the highest total already converted,
+	// read from the koreader sessions' own device_metadata. Delta-only insertion
+	// means a reinstall of the device cannot double-count, and sub-threshold
+	// deltas are never lost (they stay folded into the next push's delta).
+	GetKoreaderStatWatermark(ctx context.Context, arg GetKoreaderStatWatermarkParams) (interface{}, error)
 	GetLibrary(ctx context.Context, id pgtype.UUID) (GetLibraryRow, error)
 	GetLibraryByFolder(ctx context.Context, folderPath string) (GetLibraryByFolderRow, error)
 	GetLibraryByFolderPathPrefix(ctx context.Context, folderPath string) (GetLibraryByFolderPathPrefixRow, error)