fix(firmware): reboot on FATAL failures + emit NTP_SYNC + server-coord warning

- Config-load and camera-init FATAL branches now reboot (3s LED signal
  before restart) instead of hanging forever. Matches the enum name
  REBOOT_FATAL_* and makes camera-init failures diagnosable via the
  next boot's heartbeat recent_events. Config failures produce a
  visible reboot loop rather than a silent hang.
- Emit EVT_NTP_SYNC(seconds_since_boot) on the first NTP-synced
  reporter iteration so slow / failed NTP sync is a visible signal in
  the heartbeat's recent_events window.
- README "Deploying firmware 1.1" now opens with a "Before you flash"
  warning directing the operator to land server-side heartbeat
  schema changes first (migration 005 + stub integration) to avoid a
  strict-schema 4xx reboot loop after deployment.

README.md

@@ -200,6 +200,28 @@ python tools/serial_monitor.py --port /dev/ttyUSB0 --reset --timestamp --seconds
 
 ## Deploying firmware 1.1 (network resilience)
 
+### Before you flash
+
+Firmware 1.1 adds five new fields to the `POST /api/v1/heartbeat` payload
+(`reset_reason`, `heap_free`, `heap_min_free`, `last_disconnect_code`,
+`recent_events`). **The real server must accept these optional fields before
+you deploy firmware 1.1**, or strict-schema validation will 4xx every
+heartbeat; after 6 consecutive misses (~6h) the heartbeat-miss watchdog
+will reboot the device, producing a reboot loop.
+
+Reference migration and handler code for the real server are in this repo:
+
+- `server/heartbeat_diagnostics_stub.py` — Pydantic model extensions,
+  `store_heartbeat_diagnostics()` helper, and `EVENT_TAG_DECODER` /
+  `REBOOT_REASON_DECODER` reference tables.
+- `server/migrations/005_heartbeat_diagnostics.sql` — adds five nullable
+  columns to the `heartbeats` table (adjust table name to match the real
+  server's schema).
+
+Copy the stub additions into the production server repo, run the
+migration, and confirm a v1.1.0-shape heartbeat returns 200 before you
+flash any device.
+
 ### Flash command
 
 ```bash