feat(chat): integrate Langfuse tracing, prompt management & generation tracking

- shared/config.py: add LANGFUSE_SECRET_KEY, LANGFUSE_PUBLIC_KEY, LANGFUSE_HOST
- services/chat/app/tracing.py: new module — Langfuse client singleton,
  create_trace(), get_langfuse_callback(), get_prompt(), link_prompt_to_trace(),
  score_trace(), flush/shutdown helpers. Gracefully no-ops when keys are missing.
- services/chat/app/llm.py: add callbacks param to get_llm() for LangChain
  callback handler injection
- services/chat/app/deep_agent.py: accept langfuse_handler in all run_* and
  _run_single_agent* functions, pipe callbacks to LLM calls, fetch managed
  prompts from Langfuse with fallback to hardcoded system prompts
- services/chat/app/redis_consumer.py: create Langfuse trace per request
  (home_request/floating_request), pass callback handler to deep_agent,
  link prompt name to trace, attach output preview, flush after each request
- services/chat/app/main.py: shutdown Langfuse client in lifespan teardown
- services/chat/requirements.txt: add langfuse>=2.0.0

Langfuse prompt names: 'home_system', 'floating_system' — create these in
the Langfuse dashboard to manage prompts. Without them, hardcoded defaults
are used transparently.

services/chat/app/main.py

@@ -6,8 +6,15 @@ streams responses back via Redis pub/sub to WS Gateway.
 Owns: memory_core, memory_associative, memory_episodic, memory_proactive tables.
 """
 
+import sys
 from contextlib import asynccontextmanager
 import logging
+from pathlib import Path
+
+# Ensure the repo root is on sys.path so "shared" is importable in local dev.
+_repo_root = str(Path(__file__).resolve().parents[3])
+if _repo_root not in sys.path:
+    sys.path.insert(0, _repo_root)
 
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
@@ -31,6 +38,10 @@ async def lifespan(app: FastAPI):
     yield
     consumer_task.cancel()
 
+    from app.tracing import shutdown as shutdown_langfuse
+
+    shutdown_langfuse()
+
     from shared.db import engine
 
     await engine.dispose()