feat(sync): unlinked-books queue — ingest, page route, dashboard banner

The identification queue for device files the sync cannot match is now
live end-to-end. Previously the whole feature was dark: unmatched device
files were silently dropped (losing their reading progress), the queue
table sat empty by construction, and the page template had no route.

- Ingest: the KOReader sync loop, on an unresolvable file, upserts
  unlinked_books (one row per device + content id; re-syncs bump
  last_seen_at and refresh device-reported metadata) instead of
  dropping the file. Schema gains the (device_id, content_id) unique
  index the upsert needs.
- Page: GET /unlinked-books routes the existing page template, listing
  the caller's own unresolved rows (ListUnresolvedUnlinkedBooksForUser
  — devices.user_id scoping; no cross-user leakage).
- Dashboard banner: a warning card above the shelves, rendered only
  when the caller's queue is non-empty (hidden-unless-needed), linking
  to the page.
- Resolve flow unchanged and already gated: MatchingHandler.LinkBook
  verifies device ownership and target-item visibility (403s) and
  creates the device file alias, so the next sync matches by path.
  SyncHandler.LinkBook and Handler.LinkBook are unrouted dead handlers
  — triage candidates, not part of the live flow.

database/schema/schema.sql

@@ -1353,6 +1353,8 @@ CREATE TABLE IF NOT EXISTS unlinked_books (
 
 CREATE INDEX IF NOT EXISTS idx_unlinked_books_device ON unlinked_books(device_id);
 CREATE INDEX IF NOT EXISTS idx_unlinked_books_content_id ON unlinked_books(content_id);
+-- One queue row per (device, document): the sync ingest upserts on this pair.
+CREATE UNIQUE INDEX IF NOT EXISTS idx_unlinked_books_device_content ON unlinked_books(device_id, content_id);
 CREATE INDEX IF NOT EXISTS idx_unlinked_books_resolved ON unlinked_books(resolved);
 
 -- Create saved_filters for users to be able to filter different parts of the application