refactor: simplify mqtt pipeline state and remove event table

backend/README.md

@@ -176,7 +176,8 @@ Once you have the MJML extension installed, you can create a new email template
 This project includes a two-stage MQTT pipeline:
 
 - `mqtt-ingestor` subscribes to `terminal/{device_id}/raw` and stores payload bytes losslessly in `terminal_message_raw`.
-- `mqtt-parser-worker` consumes pending rows and writes parsed events/state.
+- `mqtt-parser-worker` consumes unparsed rows (`parsed_at IS NULL` and `parse_error IS NULL`) and updates `terminal_device_state`.
+- Parse is single-pass by design in the current implementation: failed rows keep `parse_error` and are not auto-retried with backoff.
 
 ### Run locally with Docker Compose
 
@@ -199,7 +200,6 @@ $ PYTHONPATH=. ../.venv/bin/pytest tests/mqtt --confcutdir=tests/mqtt -q
 2. Publish a sample message to `terminal/device-001/raw`.
 3. Query PostgreSQL tables:
    - `terminal_message_raw`
-   - `terminal_observation_event`
    - `terminal_device_state`
 
 ### Archive run (manual trigger)