feat(mcp): OAuth so claude.ai and ChatGPT can connect as connectors

MindSift becomes the OAuth authorization server for its own MCP
endpoint: discovery (RFC 9728/8414), dynamic client registration,
authorization code with PKCE (S256), rotating refresh tokens with reuse
detection. The consent page lives in the app at /oauth/authorize. A
grant is an api_keys entry (kind 'oauth'), so scopes, origin badges,
renaming and revoking work as for keys; tokens are valid for /mcp only.
Unused connections expire after OAUTH_IDLE_DAYS (default 90). Requires
BASE_URL; off in demo mode. Release 0.51.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/apikey_access.go

@@ -17,6 +17,8 @@ import (
 type apiKeyAuth struct {
 	ID     string
 	Scopes []string
+	// OAuth: an access token of an OAuth connection (oauth.go), valid for the MCP endpoint only
+	OAuth bool
 }
 
 // apiActor: an API key acting on entries; via is storage.OriginAPI or storage.OriginMCP. Recorded as