refactor(server): extract the song routes into routers/song.py (R3) (#864)

Upload/delete, the catalog-metadata write-back, user-meta, overrides, gap-fill,
and the per-song info payload (11 routes) move to lib/routers/song.py with their
exclusive helpers (the atomic upload commit + the song-IO lock, the upload caps,
the gap-fill proposal builders). Bodies verbatim except @app->@router and the
seam reads: meta_db->appstate.meta_db, art_override_paths->appstate.art_override_paths,
and the scan/ingest helpers that stay in server.py (the scan lifecycle owns them)
-> new appstate seam callables: kick_scan, invalidate_song_caches, stat_for_cache,
and scan_status() (a getter — the underlying dict is reassigned). The gap-fill
MBID/ISRC regexes are reached as enrichment.X; _MULTIPART_OVERHEAD_SLACK (shared
with the staying AcoustID-identify route) moves to lib/enrichment.py beside
_ACOUSTID_MAX_UPLOAD_BYTES.

ROUTE ORDER: song_router mounts AFTER art_router — get_song_info's catch-all
`/api/song/{filename:path}` would otherwise shadow `/api/song/{path}/art*`
(Starlette matches first-registered; the :path converter is greedy).

server.py: 4,478 -> 3,692 (-786).

Verified: pyflakes clean; ORDERED route table preserves the specific-before-catch-all
invariant; full pytest 2397 passed (incl the art/cover 304 + CAA-fetch tests that
caught the shadowing before it was fixed). eslint 0.

BEHAVIORAL — needs an on-device pass (upload a sloppak, edit metadata write-back,
delete a song) before merge.

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

lib/appstate.py

@@ -102,6 +102,14 @@ art_safe_name = None
 # The canonical settings-defaults builder — stays in server.py (shared with the
 # scan/artist-links code) but the settings router calls it through the seam.
 default_settings = None
+# Scan/ingest seam for the song routes (routers/song.py). kick_scan/
+# invalidate_song_caches/stat_for_cache stay in server.py (scan lifecycle owns
+# them); scan_status is a GETTER (the underlying dict is reassigned, so a value
+# would go stale) — call appstate.scan_status() to read the live status.
+kick_scan = None
+invalidate_song_caches = None
+stat_for_cache = None
+scan_status = None
 
 _SLOTS = frozenset({
     "meta_db", "audio_effect_mappings", "tuning_providers",
@@ -111,6 +119,7 @@ _SLOTS = frozenset({
     "running_version",
     "art_cache_dir", "song_pack_art_exists", "art_override_paths", "art_safe_name",
     "default_settings",
+    "kick_scan", "invalidate_song_caches", "stat_for_cache", "scan_status",
 })