fix(packaging): make mcp an optional backend and install doc backends in dev

Defer the mcp import in MCPServer.__init__ so 'import docforge' no longer requires the optional mcp dependency (matching the lazy MkDocs import). Add doc-forge[mkdocs,mcp] to the dev extra so the standard quality gate installs the backends the test suite needs.

docforge/servers/mcp_server.py

@@ -23,8 +23,6 @@ import json
 from pathlib import Path
 from typing import Any, Literal
 
-from mcp.server.fastmcp import FastMCP
-
 
 class MCPServer:
     """
@@ -54,6 +52,8 @@ class MCPServer:
             name (str):
                 Identifier used for the MCP server instance.
         """
+        from mcp.server.fastmcp import FastMCP
+
         self.mcp_root = mcp_root
         self.app = FastMCP(name)