feat: add Subsonic playlist sync source

[BlackJocker1995]

Add a new SubsonicManager module that syncs playlists from Subsonic-compatible servers (Navidrome, Airsonic, Gonic, original Subsonic) to the iPod. No tracks are downloaded — this is a playlist-only sync that references songs already in the iPod library by db_track_id, using fuzzy (title, artist) matching with curly-quote normalisation.

Architecture (mirrors PodcastManager pattern)
SubsonicManager/
client.py OpenSubsonic REST API client (token+salt auth)
plan_builder.py Builds SyncPlan with playlists_to_add/_to_edit
mapping.py Subsonic song → PCTrack mapping (unused, retained)
artwork.py getCoverArt + Navidrome placeholder detection

Settings & UI
Settings → Subsonic page: server URL/user/password, Test Connection,
per-playlist checkbox selection with Refresh button.
Sidebar: Sync Subsonic button.
Sync-time mapping dialog (QTabWidget, two tabs):
Tab 1 Mapping — choose which iPod playlist each Subsonic list
maps to (New / existing playlist).
Tab 2 Sync Mode — Remote→Local (Overwrite), Remote→Local (Add).
Local→Remote modes shown greyed out for future Subsonic write
support.

Sync flow
User clicks Sync Subsonic → dialog fetches playlist names →
user chooses mapping & mode → SubsonicPlanWorker builds plan
(playlists_to_add for new, playlists_to_edit for mapped with
merge/overwrite) → Sync Review preview → execute writes playlists
only (no file transfers — to_add is always empty).

Key design decisions

• Playlist-only: no song download; references existing iPod tracks.
• Fuzzy match: _match_key normalises curly quotes, case, punctuation.
• Merge semantics: positive playlist_mappings id = merge (keep existing
  • add new); negative id = overwrite (replace entirely).
• Deterministic playlist ids: blake2b(subsonic_id + name) → stable across re-syncs.
• Credentials stored in plaintext settings.json (matches Last.fm convention).

Tests: 123 pass (49 Subsonic-specific + 74 regression). ruff clean.