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/main.py

@@ -28,6 +28,7 @@ import math
 import os
 import time
 from collections import deque
+from contextlib import asynccontextmanager
 from pathlib import Path
 from typing import Optional, Deque
 
@@ -36,9 +37,18 @@ import numpy as np
 from fastapi import FastAPI, HTTPException
 from pydantic import BaseModel
 
+import nats_consumer
 from prompts import get_prompt
 
-app = FastAPI(title="oO ML Serving", version="1.0.0")
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    await nats_consumer.start(STATE_DIR)
+    yield
+    await nats_consumer.stop()
+
+
+app = FastAPI(title="oO ML Serving", version="1.0.0", lifespan=lifespan)
 
 LITELLM_URL = os.getenv("LITELLM_URL", "http://localhost:4000")
 LITELLM_MASTER_KEY = os.getenv("LITELLM_MASTER_KEY", "sk-oo-dev")
@@ -315,7 +325,13 @@ class GenerateResponse(BaseModel):
 
 @app.get("/health")
 def health():
-    return {"ok": True}
+    return {
+        "ok": True,
+        "nats": {
+            "enabled": bool(nats_consumer.NATS_URL),
+            "consumers": nats_consumer.consumer_health,
+        },
+    }
 
 
 _RETRY_SUFFIX = (