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

pyproject.toml

@@ -3,11 +3,10 @@ name = "proxmox-mcp-custom"
 version = "0.1.0"
 description = "Custom Proxmox MCP Server"
 dependencies = [
-    "mcp",
+    "fastmcp>=2.0",
     "proxmoxer",
     "requests", # Required for proxmoxer https backend
     "uvicorn",
-    "fastapi",
     "python-dotenv", # For local development
 ]
 requires-python = ">=3.11"