feat(ml): JetStream durable consumers in ml/serving (#98)

Adds a NATS JetStream consumer to ml/serving so the feature pipeline
can react to events without the API triggering every read.

- nats_consumer.py: durable push consumers for signals.> and feedback.>
  streams; acks on success, naks for redeliver, up to NATS_MAX_DELIVER
  attempts; per-consumer health state (last_msg_ts, processed, errors)
- main.py: FastAPI lifespan wires start/stop; /health exposes nats state
- requirements.txt: adds nats-py>=2.9.0
- Dockerfile.ml: copy all *.py from ml/serving (was missing prompts.py)

Handled subjects:
  signals.task.synced   → writes per-user sync metadata to STATE_DIR
  signals.tip.feedback  → logged for observability (reward via HTTP path)

Config: NATS_URL (empty = disabled), NATS_DURABLE_PREFIX, NATS_MAX_DELIVER

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ml/serving/requirements.txt

@@ -4,3 +4,4 @@ pydantic==2.10.4
 numpy>=1.26.0
 httpx>=0.27.0
 anthropic>=0.40.0
+nats-py>=2.9.0