feat: add FastAPI skeleton for LLM chat service

- POST /chat endpoint with message and conversation_id support
- GET /health endpoint for Cloud Run health checks
- Local and Remote LLM adapters with async httpx
- Pydantic schemas and environment-based config
- Dockerfile configured for Cloud Run deployment

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,6 @@
+# LLM Mode: "local" or "remote"
+LLM_MODE=local
+
+# Remote LLM Configuration (required if LLM_MODE=remote)
+LLM_REMOTE_URL=https://your-llm-service.com/generate
+LLM_REMOTE_TOKEN=