feat: add /health endpoint and switch to Streamable HTTP transport

- Switch from legacy SSE transport (sse_app) to Streamable HTTP (http_app)
  - Fixes 405 Method Not Allowed for MCP clients like Gemini CLI
  - MCP endpoint now at /mcp (POST) instead of /sse (GET)

- Add /health endpoint that bypasses MCP_ALLOWED_HOSTS validation
  - Returns {"status": "ok"} (200) or {"status": "degraded"} (503)
  - Enables Docker health checks without exposing to external hosts

- Add curl to Docker image for health checks
- Add healthcheck config to docker-compose files
- Add test-health and test-mcp Makefile targets
- Update documentation

Dockerfile

@@ -15,6 +15,10 @@ RUN uv pip install --system .
 # Stage 2: Final image
 FROM python:3.11-slim-bullseye
 
+# Install curl for health checks
+RUN apt-get update && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+
 WORKDIR /app
 
 # Copy installed dependencies from builder