Add MCP presets, admin connector policies and connector attribution

Six vendor-run MCP servers (Notion, Linear, Atlassian, Sentry, Asana,
Stripe) join the catalog as presets from docsgpt/connectors/presets/
mcp.yaml; existing connections to those servers show under them.

connector_policies lets an admin turn a connector off or force whose
account every share uses, and app_metadata's
connectors.allow_custom_mcp turns custom MCP servers off. Both are
enforced on the server: new connections, OAuth sign-ins, MCP test and
save, and the tools of a disabled connector stop resolving.
GET/PUT /api/admin/connectors reads and changes them.

A shared tool's owner picks owner or member credentials with
PUT /api/connections/tools/<id>/credential-mode, and a member running
the owner's account always confirms write actions.

Tool-call events and retrieved chunks name the connector they came
from (key and display name, never the account), and tool calls keep
those fields when the conversation is reloaded.

docsgpt/api/admin/__init__.py

@@ -1,5 +1,6 @@
 from .routes import admin_ns
 from . import quotas  # noqa: F401  (registers the quota resources on admin_ns)
 from . import activity  # noqa: F401  (registers the activity resources on admin_ns)
+from . import connectors  # noqa: F401  (registers the connector policy resources on admin_ns)
 
 __all__ = ["admin_ns"]