feat: add /health endpoint bypassing transport security

- Migrate from SSE to HTTP transport using fastmcp>=2.0
- Add /health endpoint for Docker health checks and load balancers
- Remove MCP_ALLOWED_HOSTS (no longer needed with http_app approach)
- Add lifespan handler for proper task group initialization
- Install curl in Docker image for health checks
- Update Makefile with test-health and test-mcp targets
- Update documentation to reflect new endpoint structure

Fixes: Health check fails with 421 Misdirected Request when
MCP_ALLOWED_HOSTS doesn't include localhost

docker-compose.yml

@@ -2,13 +2,15 @@ services:
   proxmox-mcp:
     image: gitea.ext.ben.io/b3nw/proxmox-mcp-custom:latest
     container_name: proxmox-mcp
-    environment:
-      # --- MCP Transport Security ---
-      # Allowed Host headers (comma-separated, supports :* for wildcard ports)
-      - MCP_ALLOWED_HOSTS=proxmox-mcp.example.io,localhost:*,127.0.0.1:*
     volumes:
       # Mount your clusters.json configuration file
       - ./clusters.json:/app/clusters.json:ro
     ports:
       - "8000:8000"
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 10s
     restart: unless-stopped