Fix background sync: hold BLE connection alive + persistent flusher + honest upload count

[abdulsaheel]

iOS only ever synced on app-open: pauseForBackground disconnected the band, so iOS dropped the Bluetooth assertion and suspended the app in ~34s, and the restore central never armed. Now we KEEP the live connection in the background (iOS resumes us per BLE notification under bluetooth-central) so the drain + upload continue.

• Add one connection-independent flusher (~15s) that uploads queued records and retries anything a prior tick failed to send (uploader retains rows on any non-200). Removes the per-record flushing that was tripping the backend rate limit. No cooldown.
• Show the honest uploaded count: read received from the ingest response (the client was reading a non-existent processed field, so it always showed 0/N).
• BleRestoreManager is recovery-only: event-driven re-arm via setOwnsBand, and the 50-min cooldown timer is gone (replaced by an idle-after-sync flag).
• Remove the 15-min WorkManager periodic task (iOS + Android) and all its plumbing; Android keep-alive is the Edge Tracking foreground service, iOS is the kept-alive connection with CB state-restoration as recovery only.
• Ensure the flusher + Android foreground service start on every session-establish path (init/login/reconnect/resume), not just openSession.
• Cleanup: drop the workmanager dependency, the BGTask/processing/fetch Info.plist entries, and RECEIVE_BOOT_COMPLETED.