Add atproto OAuth service with SQLite-backed storage adapters
Build AtprotoOAuthService wrapping NodeOAuthClient with state store
(AuthState model) and session store (Account model). Expose authorize,
callback, getAgent, revoke, and getClientMetadata methods. Register
as singleton via provider. Tests cover storage CRUD and metadata shape.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>