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>

infra/docker/Dockerfile.ml

@@ -2,5 +2,5 @@ FROM python:3.12-slim
 WORKDIR /app
 COPY ml/serving/requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
-COPY ml/serving/main.py .
+COPY ml/serving/*.py .
 CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]