Add opt-in ListenBrainz scrobbling

[lostf1sh]

What

Re-introduces public scrobbling as a strictly opt-in ListenBrainz integration, reversing the documented removal (PRIVACY.md, docs/FDROID.md, CHANGELOG.md are updated in the same change).

• Opt-in, default off. Nothing is submitted until a user pastes their ListenBrainz token on the Accounts screen. The token is validated on connect and stored in EncryptedSharedPreferences (same pattern as Navidrome/Jellyfin credentials), and both prefs files are excluded from Auto Backup and device transfer. Disconnecting deletes any queued listens.
• Listen threshold per the ListenBrainz spec: 4 minutes or half the track, whichever is lower, evaluated on pause-aware accumulated listening time in ListeningStatsTracker.finalizeCurrentSession() — independent of the existing stats floor and of the Subsonic track-end scrobble.
• Offline queue: new Room table listenbrainz_pending_listens (MIGRATION_2_3, idempotent per the migration convention; schema v3 exported). Rows snapshot track metadata at enqueue and are drained oldest-first by a HiltWorker with a network constraint and exponential backoff. A batch-level 400 falls back to individual submission so one bad listen can't wedge the queue; 401 pauses flushing until reconnect (listens keep queueing); 429 honors Retry-After.
• Per-source toggles for local files, Subsonic, and Jellyfin playback (all on once connected), so users whose server already scrobbles server-side can avoid duplicates.
• Playing-now is submitted on track start/resume (debounced) with duration_ms, relying on ListenBrainz's server-side expiry instead of a heartbeat.
• Groundwork for MusicBrainz tag lookup: songs.mb_recording_id / mb_release_id / mb_artist_id columns (not yet populated); scrobbles will carry recording MBIDs once tag reading lands.

Why

Lets users keep their listening history on their own terms while preserving the app's privacy posture: consent is explicit (token paste), revocation is complete (disconnect clears the queue), and the F-Droid listing requirement is reframed to strictly-opt-in rather than absent.

UI changes

The Accounts screen gains a ListenBrainz card (token dialog with masked input, connection state, queued-listen count, reconnect flow for invalidated tokens, per-source toggles) and a new "Available services" section so services can be connected while others are already linked — previously connect buttons only appeared when no account was connected. Screenshots to follow.

Verification

• :app:compileDebugKotlin — passes
• :app:testDebugUnitTest — passes (new ScrobbleManagerTest covers the threshold edges; ListeningStatsTrackerTest updated for the new dependency)
• :app:lintDebug — passes

An adversarial review pass was done on the change; its findings (disconnect/enqueue race, missing backup exclusions, missing reconnect path, 401/429 handling, unmasked token field) are all fixed in this branch. Known gap: no unit tests yet for the disconnect-ordering and 401-pause behaviors — they need fakes for EncryptedSharedPreferences/WorkManager.