Add Langfuse observability: traces, prompt management, prompt-to-generation linking

- New app/core/langfuse_client.py: lazy singleton client, get_prompt_or_fallback()
  helper (returns raw template + prompt obj for linking), extract_usage() for token
  counts. No-ops when LANGFUSE_* env vars are not set.
- deep_agent.py: home-agent and floating-agent runs wrapped in spans; each ainvoke
  wrapped in a generation with model/input/output/usage; prompts fetched from
  Langfuse (adiuva-home-agent, adiuva-floating-agent, adiuva-floating-classifier)
  with hardcoded fallback.
- agent_runner.py: step1-classifier and step2-processor LLM calls traced; batch
  agent _run_agent_with_tools spans + generations; cloud-processor included.
  Prompts: adiuva-step1-classifier, adiuva-step2-processor, adiuva-cloud-processor.
- agent_setup.py: journey-setup span + generation per ainvoke; prompt_obj stored
  on JourneySession and reused across turns. Prompt: journey_system.
- settings.py: LANGFUSE_SECRET_KEY, LANGFUSE_PUBLIC_KEY, LANGFUSE_HOST added.
- .env.example: Langfuse section with EU/US/self-hosted host comments.
- requirements.txt: langfuse>=2.0.0.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -39,6 +39,13 @@ QDRANT_URL=
 QDRANT_API_KEY=
 # For local Qdrant (homelab): QDRANT_URL=http://qdrant:6333
 
+# ── Langfuse (leave empty to disable observability) ───────────────────────────
+LANGFUSE_SECRET_KEY=
+LANGFUSE_PUBLIC_KEY=
+# LANGFUSE_HOST=https://cloud.langfuse.com        # EU (default)
+# LANGFUSE_HOST=https://us.cloud.langfuse.com     # US
+# LANGFUSE_HOST=http://localhost:3000             # Self-hosted
+
 # ── CORS ──────────────────────────────────────────────────────────────────────
 # Comma-separated list parsed by Settings (override default if needed)
 # CORS_ORIGINS=["app://.","http://localhost:3000"]