feat(library): smart collections as a library provider (#641)

Implements feedBack#636 item 2 (P1) — saved library filters that stay live,
the homelab primitive FeedBack was missing (Plex smart collections / Navidrome
.nsp / *arr custom filters).

A collection is a saved /api/library query surfaced as a registered library
provider, so it appears in the v3 source picker and inherits the whole Songs
UI (paging, stats, A–Z rail, art) with no new screen.

- Storage reuses the playlist subsystem: a `playlists.rules` JSON column
  (additive, idempotent migration). A row with rules != NULL is a smart
  collection; list_playlists + get_playlist filter `rules IS NULL`, so
  collections are excluded from the manual-playlist list and read-only to
  every playlist mutation that gates on get_playlist.
- SmartCollectionProvider (kind="local" — matched songs are local rows, so the
  client's play/art paths stay on the local branch) delegates query_page/
  query_stats/query_artists to the local DB with the stored rules applied;
  tuning_names/get_art delegate straight through. Registered via a boot scan +
  on create/update (replace=True) / delete.
- Rules mirror the raw /api/library query params; `_sanitize_collection_rules`
  drops unknown keys and is applied at API ingress AND on provider load, so a
  hand-edited / imported bad value can't crash a query.
- API: GET/POST/PUT/DELETE /api/collections. Frontend: a "＋ Save as
  collection" action in the v3 filter drawer (local provider + active filters
  only) that names the current filter set and switches to it.

Reviewed by Codex; 3 findings fixed (local-kind playback path, save gated to
local provider, re-sanitize persisted rules).

Tests: tests/test_collections_api.py (CRUD, provider filtering, restart
re-registration, kind=local, corrupt-rule tolerance, playlist isolation),
tests/js/v3_collections.test.js.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 - **Keyset (cursor) pagination for the library grid — the data layer for an upcoming virtualized grid, and a latent paging bug fixed along the way.** Every library sort now carries a unique `filename` tiebreak, making the order **total** — which fixes a latent bug where rows sharing a sort key (e.g. two songs by the same artist) could be skipped or duplicated across `OFFSET` pages. `GET /api/library` gains an opaque `after` cursor + a `next_cursor` in the response: passing the cursor back fetches the next page with a **WHERE-seek** instead of `OFFSET`, so deep paging is O(page) regardless of depth. The seek is NULL-aware and exactly `OFFSET`-equivalent (verified across artist/title/recent, ascending + descending, including the legacy `dir=desc` shape and NULL sort keys); unknown/compound sorts and bad cursors fall back to `OFFSET`, and only the local provider is handed a cursor (collections/remote page by `OFFSET`). New composite `(artist NOCASE, filename)` / `(title NOCASE, filename)` / `(mtime, filename)` indexes cover the order. This is stage 1 of the virtualized-grid project (got-feedback/feedBack#636 item 3); the DOM-recycling render window builds on it next. Tests: `tests/test_library_keyset.py` (keyset==OFFSET parity, stable tiebreak, dir=desc, NULL keys, cursor fallback).
+- **Smart collections — save a set of library filters as a live, auto-updating source.** A collection is a saved `/api/library` query (e.g. "Drop-D tunings", "sloppak only", "recently added") that stays live: it's registered as a **library provider**, so it shows up in the v3 Songs source picker and inherits the whole grid UI — paging, stats, the A–Z rail, art — for free, with **no new screen**. Storage reuses the playlist subsystem (a `playlists.rules` JSON blob = a smart collection; membership is the live filter result, not stored songs, and collections are excluded from the manual-playlist list + read-only to playlist mutations). New `GET`/`POST`/`PUT`/`DELETE /api/collections`; a per-collection `SmartCollectionProvider` delegates `query_page`/`query_stats`/`query_artists` to the local DB with the stored rules applied; providers are re-registered from a boot scan so collections survive a restart. Rules mirror the raw `/api/library` query params (unknown keys dropped, never 500). Frontend: a "＋ Save as collection" action in the v3 filter drawer (shown when filters are active) names the current filter set and switches to it. The charrette's "the homelab primitive FeedBack was missing" pick (got-feedback/feedBack#636 item 2); richer rule fields (accuracy, genre, difficulty) follow as the metadata work lands. Tests: `tests/test_collections_api.py`, `tests/js/v3_collections.test.js`.
 - **The settings backup now includes your library database + custom art — your scores, favorites, playlists, and play history are no longer the one thing a backup can't save.** `GET /api/settings/export` gains an additive `core_server_files` section carrying a **consistent snapshot of `web_library.db`** (taken via the SQLite online-backup API, so it's a complete single file even while the server is running) plus any custom **playlist covers** and **avatar** (`CONFIG_DIR/playlist_covers/`, `CONFIG_DIR/avatars/`). On `POST /api/settings/import` the database is **staged** to `web_library.db.restore` rather than written over the live, open DB; it's swapped in at the next startup (`_apply_pending_db_restore`, before the connection opens), which also clears the old WAL sidecars so a stale `-wal` can't be replayed onto the restored file — the import response sets `restart_required: true` and warns accordingly. Custom art is written immediately. The bundle stays backward-compatible (older servers ignore the new section). Came out of the library design charrette (dev-ops lens's top "protect irreplaceable data" pick, got-feedback/feedBack#636). _Known gap:_ custom uploaded **song** art is still commingled with the rebuildable thumbnail cache in `art_cache/`, so it isn't bundled yet (a tracked follow-up). Tests: `tests/test_settings_export_library_db.py` (snapshot consistency, staged-not-live restore, sidecar clearing, traversal rejection, full round-trip).
 - **A persisted wishlist — keep a list of songs you want but don't own yet.** New `wanted` table + `GET`/`POST`/`DELETE /api/wanted` give FeedBack the *arr-style "Wanted/Monitored" primitive it was missing: an entry is a *not-owned* song (artist/title/source/source_ref/note), so it lives in its own table rather than the playlist subsystem (which references owned local files). The API is idempotent on identity (case-insensitive artist+title, plus source+source_ref), so a producer — the `find_more` ownership-diff, or a manual add — can re-post without duplicating. Newest-first. Backend primitive for the charrette's wishlist finding (got-feedback/feedBack#636 item 4); the consuming UI lives in the producing plugin. Tests: `tests/test_wanted_api.py`.
 - **Practice-aware library home — a "Repertoire" meter + a "Keep practicing" shelf on the v3 Songs page.** The library opened cold into a flat sorted grid; now the unfiltered grid front door leads with two practice-aware surfaces built entirely from data already on hand (no new endpoints or stored state). A **Repertoire meter** shows how much of your library you can actually play — *"Repertoire: 12 of 80 songs · 7 in progress"* with a progress bar — counting songs at or above the same mastery threshold the green accuracy badge uses (≥ 90% best accuracy) over the unfiltered library total. A **"Keep practicing" shelf** is a horizontal row of your recently-played-but-not-yet-mastered songs (newest first, click to play) — the practice-accuracy-driven "continue" rail a media server can't do. Both reuse `/api/stats/best` (already loaded for the card badges) + `/api/stats/recent`; they show **only** on the grid view when you aren't searching/filtering/selecting, refresh after a song is scored, and collapse to nothing on an empty library. Soft-gamification only — descriptive encouragement (goal-gradient / endowed-progress), never content-gating, decay, or nagging. Frontend-only: `static/v3/songs.js` (`renderLibraryHome`/`_repertoireCounts`), `static/v3/v3.css`. Came out of the library design charrette (the UX + gamification lenses' top pick). Tests: `tests/js/v3_keep_practicing.test.js`.