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>

Dockerfile

@@ -0,0 +1,16 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+# Install dependencies
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application code
+COPY app/ ./app/
+
+# Expose port for Cloud Run
+EXPOSE 8080
+
+# Run the application
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8080"]