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/providers/credentials.py

@@ -14,6 +14,7 @@ from app.config import get_settings
 
 _CREDENTIAL_ID = re.compile(r"^[A-Za-z0-9][A-Za-z0-9._-]{0,127}$")
 _PLUGIN_CREDENTIAL_PREFIX = "plugin."
+_MCP_CREDENTIAL_PREFIX = "mcp."
 
 
 class CredentialStoreError(RuntimeError):
@@ -27,10 +28,10 @@ class CredentialResolver(Protocol):
 def validate_provider_credential_id(credential_id: str | None) -> None:
     """阻止 Provider 和通用凭据 API 跨入 Plugin 私有命名空间。"""
 
-    if credential_id and credential_id.casefold().startswith(
-        _PLUGIN_CREDENTIAL_PREFIX
-    ):
+    if credential_id and credential_id.casefold().startswith(_PLUGIN_CREDENTIAL_PREFIX):
         raise CredentialStoreError("Credential namespace is reserved for Plugin settings.")
+    if credential_id and credential_id.casefold().startswith(_MCP_CREDENTIAL_PREFIX):
+        raise CredentialStoreError("Credential namespace is reserved for MCP settings.")
 
 
 class EnvironmentCredentialResolver: