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.

internal/database/querier.go

@@ -48,6 +48,8 @@ type Querier interface {
 	CountCollectionLiveItems(ctx context.Context, arg CountCollectionLiveItemsParams) (int32, error)
 	// Count unlinked books for a device
 	CountUnlinkedBooks(ctx context.Context, deviceID pgtype.UUID) (int64, error)
+	// Banner count: unresolved queue rows across the caller's own devices.
+	CountUnresolvedUnlinkedBooksForUser(ctx context.Context, userID pgtype.UUID) (int64, error)
 	CountUserDevices(ctx context.Context, userID pgtype.UUID) (int64, error)
 	CreateAutoResolvedSyncConflict(ctx context.Context, arg CreateAutoResolvedSyncConflictParams) (SyncConflicts, error)
 	// COLLECTIONS QUERIES
@@ -469,6 +471,8 @@ type Querier interface {
 	ListSyncConflictsByUser(ctx context.Context, userID pgtype.UUID) ([]ListSyncConflictsByUserRow, error)
 	// List unresolved unlinked books with pagination
 	ListUnresolvedUnlinkedBooks(ctx context.Context, arg ListUnresolvedUnlinkedBooksParams) ([]ListUnresolvedUnlinkedBooksRow, error)
+	// Page list: unresolved queue rows for the caller's own devices.
+	ListUnresolvedUnlinkedBooksForUser(ctx context.Context, userID pgtype.UUID) ([]ListUnresolvedUnlinkedBooksForUserRow, error)
 	ListUsers(ctx context.Context) ([]ListUsersRow, error)
 	// First consecutive scan that cannot find the file on disk.
 	MarkMediaItemMissing(ctx context.Context, id pgtype.UUID) error
@@ -607,6 +611,9 @@ type Querier interface {
 	UpsertPanelData(ctx context.Context, arg UpsertPanelDataParams) (PanelData, error)
 	UpsertReaderSettings(ctx context.Context, arg UpsertReaderSettingsParams) (ReaderSettings, error)
 	UpsertSystemSetting(ctx context.Context, arg UpsertSystemSettingParams) (SystemSettings, error)
+	// Ingest: insert or refresh the queue row for a device file that could not
+	// be identified. One row per (device, content_id); re-syncs bump last_seen.
+	UpsertUnlinkedBook(ctx context.Context, arg UpsertUnlinkedBookParams) (UnlinkedBooks, error)
 }
 
 var _ Querier = (*Queries)(nil)