feat(mqtt): add device health telemetry and migrate to v2 topic spec

Two efforts that landed together because the v2 topic work extends
tables the health-telemetry effort added days earlier in the same
files/functions, making them impractical to separate cleanly:

Health/diagnostics telemetry (schema, Jul 13-17):
- New Postgres tables: device_alert_events, device_boot_events,
  device_ping_samples, device_diagnostics_reports, plus a `source`
  column on device_logs to distinguish log origins
- Query/service layer in pg_mqtt.py and database/__init__.py for
  inserting and listing this history, plus a "latest metrics" endpoint
  combining most-recent diagnostics + ping RTT per device
- mqtt/router.py gains list endpoints for alert/boot/ping/diagnostics
  history, consumed by the upcoming Health tab

MQTT v2 topic migration (Sep 21):
- Heartbeat payload flattened per vesper_mqtt_topic_spec_v2.md, adding
  rssi/free_heap/state/ok fields
- Command replies move to control/ack, device-initiated events to
  control/reports; mqtt/client.py subscribes to the new topic set and
  runs a ping_loop (wired up in main.py) for RTT sampling
- mqtt/logger.py and pg_mqtt.py updated to parse and persist the new
  payload shape alongside the legacy fields for backwards compatibility

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/main.py

@@ -116,6 +116,7 @@ async def startup():
     mqtt_manager.start(asyncio.get_event_loop())
     asyncio.create_task(db.partition_manager_loop())
     asyncio.create_task(db.purge_loop())
+    asyncio.create_task(mqtt_manager.ping_loop())
     asyncio.create_task(nextcloud_keepalive_loop())
     asyncio.create_task(crm_poll_loop())
     sync_accounts = [a for a in get_mail_accounts() if a.get("sync_inbound") and a.get("imap_host")]