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.

shared/config.py

@@ -76,6 +76,11 @@ class Settings(BaseSettings):
     MS_TENANT_ID: str = "common"
     OAUTH_ENCRYPTION_KEY: str = ""
 
+    # ── Langfuse (observability) ─────────────────────────────────────
+    LANGFUSE_SECRET_KEY: str = ""
+    LANGFUSE_PUBLIC_KEY: str = ""
+    LANGFUSE_HOST: str = "https://cloud.langfuse.com"
+
     # ── CORS ─────────────────────────────────────────────────────────
     CORS_ORIGINS: list[str] = ["app://.", "http://localhost:3000", "http://localhost:5173"]