feat(batch-agent): integrate Langfuse tracing

- tracing.py: init/shutdown, trace_span, get_langfuse_callback, prompt mgmt
- main.py: init_langfuse at startup, shutdown on teardown
- redis_consumer.py: trace_span around journey_start/message/agent_trigger
- agent_runner.py: thread langfuse_handler through classify + processing LLM
- journey.py: thread langfuse_handler through _call_llm_with_tools
- llm.py: accept callbacks param, forward to LLM constructors
- requirements.txt: add langfuse>=3.0.0

services/batch-agent/app/main.py

@@ -29,6 +29,10 @@ logger = logging.getLogger(__name__)
 
 @asynccontextmanager
 async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
+    # Initialise Langfuse tracing (no-op if keys are missing)
+    from app.tracing import init_langfuse
+    init_langfuse()
+
     logger.info("batch-agent: starting Redis consumer")
     task = asyncio.create_task(start_consumer())
     yield
@@ -37,6 +41,16 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
         await task
     except asyncio.CancelledError:
         pass
+
+    from app.tracing import shutdown as shutdown_langfuse
+    shutdown_langfuse()
+
+    from shared.db import engine
+    await engine.dispose()
+
+    from shared.redis import redis_client
+    await redis_client.aclose()
+
     logger.info("batch-agent: Redis consumer stopped")