feat(db): add query to count media items per visible library

Add GetVisibleLibraryMediaCounts, which returns the media item count for
each library visible to a given user in a single GROUP BY query over
media_items. It mirrors the visibility logic in GetUserVisibleLibraries
(libraries default to visible unless an explicit false row exists) so
counts can be resolved in one round-trip instead of N per-library
lookups.

Regenerated sqlc bindings (querier.go, queries.sql.go).

internal/database/querier.go

@@ -303,6 +303,7 @@ type Querier interface {
 	// Get user reading history for analytics
 	GetUserReadingHistory(ctx context.Context, arg GetUserReadingHistoryParams) ([]GetUserReadingHistoryRow, error)
 	GetUserVisibleLibraries(ctx context.Context, userID pgtype.UUID) ([]GetUserVisibleLibrariesRow, error)
+	GetVisibleLibraryMediaCounts(ctx context.Context, userID pgtype.UUID) ([]GetVisibleLibraryMediaCountsRow, error)
 	HasRecentConflictResolution(ctx context.Context, arg HasRecentConflictResolutionParams) (bool, error)
 	IncrementSyncQueueAttempts(ctx context.Context, arg IncrementSyncQueueAttemptsParams) (SyncQueue, error)
 	// Check if book is in collection