Harden realtime foreground service lifecycle

[in-jun]

Cleans up the realtime foreground service and periodic backstop worker around notifications, wake locks, and background start restrictions.

• Skip the periodic worker's foreground promotion when the realtime service is already running. In REALTIME mode the service holds its own ongoing sync_status notification (1001), so the worker promoting itself posted a second, redundant "Syncing files" notification (1002) on every backstop pass; it now checks the realtime flag and only promotes in PERIODIC mode where the long-pass execution budget is actually needed.
• Give the ongoing status notification a contentIntent. The persistent notification that REALTIME users always see in the shade was a dead tap target; it now deep-links into MainActivity, reusing the immutable PendingIntent pattern SyncAlertNotifier already uses.
• Acquire a timeout-bounded PARTIAL_WAKE_LOCK around each realtime pass. A foreground service keeps the process and network alive but does not stop CPU suspend once the screen turns off, so an in-flight transfer could freeze mid-pass until the next wakeup; the wake lock (released in a finally) keeps the pass running. This also puts the already-declared WAKE_LOCK permission to use.
• Guard startForegroundService() against ForegroundServiceStartNotAllowedException. apply() runs inside viewModelScope, which outlives the Activity, so a late awaitLoaded()/sync-lock wait could start the service from the background and crash on Android 12+; it now falls back to the WorkManager periodic backstop, matching the guard the boot path already has.

Fixes #103
Fixes #80
Fixes #79
Fixes #17

[in-jun]

Reviewed against all four referenced issues and the surrounding lifecycle code. This is clean and well-scoped — each change maps to one issue and stays within app/sync.

#103 — duplicate notification. Gating the worker's setForeground() on !isRealtimeServiceEnabled() reads the same KEY_REALTIME flag apply() writes, so the redundant 1002 notification is suppressed in REALTIME while PERIODIC keeps its extended budget. Correct.

#80 — dead tap target. contentIntent on the shared status notification reuses the immutable PendingIntent pattern from SyncAlertNotifier; request code 1001 doesn't collide with the 2000+ alert codes, and the intents are equal anyway. Good that the worker's 1002 notification becomes tappable too.

#79 — wake lock. Timeout-bounded acquire after the network-gate early return, released in finally behind the isHeld guard so the timeout auto-release can't trigger an under-lock. Passes are serialized by the collect, and cancellation still runs the finally, so there's no leak on shutdown.

#17 — background FGS start. The guard swallows only ForegroundServiceStartNotAllowedException on API 31+ and rethrows everything else, with the SDK_INT check short-circuiting ahead of the is check (safe on older APIs). This also restores deletePair's forgetAncestors cleanup, which the uncaught throw used to skip.

One behavioral note, not blocking: in REALTIME mode where the service was refused a background start, the backstop worker now also skips promotion and runs a long pass under the default budget. That's an acceptable degradation given the engine is crash-safe and recordIfStoppedMidPass surfaces a persistently stopped pair.

LGTM.