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

IMPLEMENTATION.md

@@ -8,7 +8,7 @@ Technical documentation for the Komodo MCP Server.
 ┌─────────────────────────────────────────────────────────┐
 │                    MCP Client (AI Agent)                │
 └─────────────────────────┬───────────────────────────────┘
-                          │ SSE
+                          │ HTTP (POST /mcp)
                           ▼
 ┌─────────────────────────────────────────────────────────┐
 │              Docker Container (komodo-mcp)              │