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/queries/queries.sql

@@ -565,6 +565,18 @@ SELECT * FROM media_reviews WHERE media_item_id = $1 AND user_id = $2;
 -- name: DeleteMediaReview :exec
 DELETE FROM media_reviews WHERE media_item_id = $1 AND user_id = $2;
 
+-- 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).
+-- name: GetKoreaderStatWatermark :one
+SELECT COALESCE(MAX((device_metadata->>'total_seconds')::bigint), 0) AS total_seconds
+FROM reading_history
+WHERE user_id = $1
+  AND media_item_id = $2
+  AND device_metadata->>'source' = 'koreader';
+
 -- Search Media Items queries
 -- name: SearchMediaItems :many
 SELECT mi.*, l.name as library_name, lt.name as library_type_name