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/queries/queries.sql

@@ -137,6 +137,14 @@ LEFT JOIN library_visibility lv ON l.id = lv.library_id AND lv.user_id = $1
 WHERE COALESCE(lv.is_visible, true) = true
 ORDER BY l.created_at ASC;
 
+-- name: GetVisibleLibraryMediaCounts :many
+SELECT l.id, COUNT(mi.id) as media_count
+FROM libraries l
+LEFT JOIN library_visibility lv ON l.id = lv.library_id AND lv.user_id = $1
+LEFT JOIN media_items mi ON mi.library_id = l.id
+WHERE COALESCE(lv.is_visible, true) = true
+GROUP BY l.id;
+
 -- Media Items queries
 -- name: CreateMediaItem :one
 INSERT INTO media_items (library_id, title, author, isbn, description, file_path, file_size, mime_type, cover_image_path, series, series_number, tags, tags_search, asin, date_published, publisher, contributors, contributors_search, language, edition, page_count, genre, copyright_year, goodreads_id, openlibrary_id, google_books_id, added_by_admin_id, created_at, imported_at, manga_type, reading_direction, series_count, volume, imprint, age_rating, web_url, story_arc, is_black_and_white, metadata_notes, community_rating, alternate_info, scan_information, summary, library_type_name)