feat(reliability): idempotent Kafka consumers with processed-events table

[MarkCesium]

Summary

backend/devices/src/infra/broker/routes.py:30-96 processes events without deduplication. At-least-once delivery from PlaceBrain/places#8 / outbox will cause duplicates; today on_place_deleted runs a cascade that is not safe to repeat.

Changes

• New table processed_events(event_id UUID PK, consumer TEXT, processed_at TIMESTAMPTZ).
• Each BaseEvent carries event_id (add to placebrain-contracts if missing).
• Wrap every consumer handler:

  async with uow.begin():
      if await uow.processed_events.exists(event_id, consumer_name):
          return
      await handle(payload)
      await uow.processed_events.insert(event_id, consumer_name)

• Applies to consumers on places.member.*, places.place.deleted, telemetry.status, auth.user.deleted.

Verification

• Re-deliver the same PlaceDeleted event twice (e.g. reset consumer group offset) — devices are deleted once, logs show one cascade, one DevicesBulkDeleted publish.