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/domain/models.go

@@ -634,6 +634,8 @@ type APIKey struct {
 	Scopes     string `json:"scopes"`
 	LastUsedAt string `json:"lastUsedAt,omitempty"`
 	CreatedAt  string `json:"createdAt"`
+	// Kind: "key" (secret shown once) or "oauth" (a connection agreed to on the consent page)
+	Kind string `json:"kind"`
 }
 
 type WSMessage struct {