feat(mcp): add standalone server registry

Implement C.1 stdio MCP server CRUD, encrypted environment secrets, command digest approval, connection tests, lifecycle recovery, and dynamic tool registration. Add the standalone frontend configuration center, contracts, regression tests, and development documentation.

backend/app/extensions/mcp.py

@@ -74,12 +74,14 @@ class McpStdioClient:
         command: list[str],
         *,
         cwd: Path,
+        environment: dict[str, str] | None = None,
         on_seen: Callable[[], None],
         on_broken: Callable[[str], None],
         on_tools_changed: Callable[[], None],
     ) -> None:
         self.command = command
         self.cwd = cwd
+        self.environment = environment or {}
         self.on_seen = on_seen
         self.on_broken = on_broken
         self.on_tools_changed = on_tools_changed
@@ -99,6 +101,7 @@ class McpStdioClient:
         # 平台级沙箱启动器；uvx 只隔离 Python 依赖，不能替代系统权限限制。
         creation_flags = getattr(subprocess, "CREATE_NO_WINDOW", 0) if os.name == "nt" else 0
         environment = _subprocess_environment()
+        environment.update(self.environment)
         environment.setdefault("PYTHONUNBUFFERED", "1")
         try:
             self.process = subprocess.Popen(
@@ -383,6 +386,10 @@ class McpBridge:
         package_path: Path,
         declared_permissions: list[str],
         on_unavailable: Callable[[str, str], None],
+        *,
+        command_override: list[str] | None = None,
+        environment: dict[str, str] | None = None,
+        tool_source: str = "plugin",
     ) -> list[McpDiscoveredTool]:
         if backend.transport != "stdio":
             raise McpBridgeError(
@@ -390,7 +397,7 @@ class McpBridge:
                 "Phase C only supports the MCP stdio transport.",
                 status_code=501,
             )
-        command = self._resolve_command(package_path, backend)
+        command = command_override or self._resolve_command(package_path, backend)
         now = datetime.now(timezone.utc)
         status = PluginHostStatus(
             plugin_id=plugin_id,
@@ -420,6 +427,7 @@ class McpBridge:
         client = McpStdioClient(
             command,
             cwd=package_path,
+            environment=environment,
             on_seen=seen,
             on_broken=broken,
             on_tools_changed=tools_changed,
@@ -470,7 +478,7 @@ class McpBridge:
             status.server_version = _optional_string(server_info.get("version"))
             client.notify("notifications/initialized")
             discovered = self._discover_tools(
-                plugin_id, client, backend, declared_permissions
+                plugin_id, client, backend, declared_permissions, tool_source
             )
             status.status = PluginHostState.ready
             status.tools_count = len(discovered)
@@ -601,6 +609,7 @@ class McpBridge:
         client: McpStdioClient,
         backend: PluginBackend,
         declared_permissions: list[str],
+        tool_source: str,
     ) -> list[McpDiscoveredTool]:
         discovered: list[McpDiscoveredTool] = []
         cursor: str | None = None
@@ -620,7 +629,7 @@ class McpBridge:
                 )
             for raw in raw_tools:
                 discovered.append(
-                    self._map_tool(plugin_id, raw, declared_permissions)
+                    self._map_tool(plugin_id, raw, declared_permissions, tool_source)
                 )
                 if len(discovered) > MAX_MCP_TOOLS:
                     raise McpBridgeError(
@@ -648,7 +657,10 @@ class McpBridge:
 
     @staticmethod
     def _map_tool(
-        plugin_id: str, raw: Any, declared_permissions: list[str]
+        plugin_id: str,
+        raw: Any,
+        declared_permissions: list[str],
+        tool_source: str = "plugin",
     ) -> McpDiscoveredTool:
         if not isinstance(raw, dict):
             raise McpBridgeError(
@@ -712,7 +724,7 @@ class McpBridge:
                 description=description if isinstance(description, str) else remote_name,
                 parameters=schema,
                 permission=permission,
-                source="plugin",
+                source=tool_source,
             ),
         )