Add MQTT webhook bridge with resilient MQTT connection handling

FastAPI service that subscribes to nearle/# on the MQTT broker and forwards
rider status, periodic logs, and profile updates to the Jupiter webhook API.

Includes fixes for a failure mode that took the bridge down silently for
~3.8 days in June 2026, during which it served HTTP 200 on / while
forwarding zero events:

- Retry MQTT connection with backoff instead of letting a single failed
  connect kill the background thread permanently.
- Subscribe inside on_connect so subscriptions are restored after an
  automatic reconnect; the session is clean, so a reconnect previously came
  back subscribed to nothing.
- Add /healthz returning 503 when MQTT is disconnected, so a monitor can
  distinguish a live bridge from a dead one.
- Replace thread-per-message forwarding with a bounded worker pool, which
  keeps memory flat when the upstream API stalls on its 5s timeout.
- Pin requirements.txt to the versions verified in production; it was
  unpinned and had already drifted to paho-mqtt 2.1.0.
- Set PYTHONUNBUFFERED so container logs are not block-buffered.
- Name the compose image nearle-api-image to match the deployed tag.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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