Add transactions to auth procedures and extract DB models

- Wrap multiple DB operations in transactions for atomicity:
  - login-if-completed: device upsert + session + login_request deletion
  - forgot-password: delete old tokens + insert new token
  - signup: session + email_verification creation

- Extract reusable DB model operations to packages/db/src/models/:
  - sessions.ts: insertSession()
  - user-devices.ts: upsertUserDevice(), isDeviceTrusted()

- Update session.ts to use new model functions from @reviq/db
- Fix type narrowing in admin.test.ts

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/db/src/index.ts

@@ -33,3 +33,7 @@ export {
   parseToken,
   TOKEN_PREFIX,
 } from "./helpers/token.js";
+/**
+ * Export model operations
+ */
+export * from "./models/index.js";