refactor(auth): type auth providers and decouple Google auth from disk storage

- Make MailIntakeAuthProvider generic over credential type to enforce
  typed auth contracts between providers and adapters
- Refactor Google OAuth provider to use CredentialStore abstraction
  instead of filesystem-based pickle persistence
- Remove node-local state assumptions from Google auth implementation
- Clarify documentation to distinguish credential lifecycle from
  credential persistence responsibilities

This change enables distributed-safe authentication providers and
allows multiple credential persistence strategies without modifying
auth logic.

mail_intake/credentials/__init__.py

@@ -0,0 +1,8 @@
+"""
+from mail_intake.credentials.store import CredentialStore
+
+from google.oauth2.credentials import Credentials
+
+GoogleCredentialStore = CredentialStore[Credentials]
+
+"""