feat(auth): introduce credential store abstraction and refactor Google auth

- Add generic CredentialStore abstraction for credential persistence
- Introduce pickle- and Redis-backed credential store implementations
- Refactor Google OAuth provider to delegate persistence to CredentialStore
- Make auth providers generic over credential type for stricter contracts
- Update package documentation to reflect credential lifecycle vs persistence split
- Add credentials module to public API surface
- Harden .gitignore to exclude credential and token artifacts

This release removes node-local persistence assumptions, enables
distributed-safe authentication, and formalizes credential storage as
a first-class extension point.

pyproject.toml

@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "mail-intake"
-version = "0.0.1"
+version = "0.0.2"
 description = "Structured mail ingestion and correspondence parsing with provider adapters (Gmail-first)."
 readme = "README.md"
 requires-python = ">=3.10"