feat: add /health endpoint and switch to fastmcp 2.0

- Switch from mcp package to fastmcp>=2.0 for Streamable HTTP support
  - Fixes 405 Method Not Allowed for MCP clients like Gemini CLI
  - MCP endpoint now at /mcp (POST) instead of /sse (GET)
  - Removes MCP_ALLOWED_HOSTS (handled at reverse proxy level)

- Add /health endpoint for Docker health checks
  - Returns {"status": "ok"} (200) or {"status": "degraded"} (503)
  - Enables health checks without transport security issues

- 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

docker-compose.dev.yml

@@ -6,4 +6,10 @@ services:
       - .env
     ports:
       - "8001:8000"
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 5s
     restart: unless-stopped