v3 library: genre filter facet (reads feedpak genres) (#690)

* feat(v3): genre filter facet (reads the feedpak genres field)

Adds a Genre facet to the library Filters drawer, populated from each song's
primary genre. Server: a genre column (idempotent ALTER, indexed) written from
the sloppak manifest's genres list on scan (primary = genres[0]); a genre band in
_build_where (OR within the selected set); GET /api/library/genres for the facet's
distinct list. Client: a multi-select Genre section mirroring the tuning/mastery
facets. Follows the merged spec 1.12.0 genres field (#40).

v1 stores only the PRIMARY genre (genres[0]); secondary genres aren't filterable
yet. Threaded like the mastery filter (separate query_page kwarg, so query_artists
/query_stats are unaffected) -- genre filters the grid view. Needs a rescan to
backfill genre on existing packs (only packs whose manifest carries genres).

Verified live: a sloppak tagged genres:[Metal, Rock] -> /api/library/genres
returns [Metal]; ?genre=Metal returns it; ?genre=Rock (secondary) returns none; a
plain song stays ungenred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QbexxfTt8q2tAn436MqGWF

* fix(v3): scope genre facet to local provider (PR #690 review)

The /api/library/genres facet always read the local meta DB, so a remote
provider showed local genres while the `genre` filter was a no-op on that
provider's grid. Make the endpoint provider-aware: return an empty facet
for remote providers (kind != "local") and keep serving genres for the
local library and its smart collections, which share the local DB. The
v3 client now passes the active provider, mirroring the tuning-names facet.

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

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

lib/sloppak.py

@@ -919,6 +919,8 @@ def extract_meta(path: Path) -> dict:
         "artist": str(manifest.get("artist", "")),
         "album": str(manifest.get("album", "")),
         "year": str(manifest.get("year", "") or ""),
+        # Primary genre from the feedpak `genres` list (spec 1.12.0); [0] = primary.
+        "genre": (lambda g: str(g[0]) if isinstance(g, list) and g else "")(manifest.get("genres")),
         "duration": float(manifest.get("duration", 0) or 0),
         "tuning_offsets": tuning_offsets,  # caller maps to a name via tunings.tuning_name
         "arrangements": arrangements,