docs(api): update package docs for credential store–based auth design

- Update package-level documentation to reflect credential persistence
  as a first-class concept
- Replace token_path-based examples with CredentialStore usage
- Add credentials module to documented public API surface
- Clarify auth vs credential persistence responsibilities
- Align design guarantees with distributed-safe authentication model

mail_intake/credentials/__init__.py

@@ -1,8 +1,29 @@
 """
+Credential persistence interfaces and implementations for Mail Intake.
+
+This package defines the abstractions and concrete implementations used
+to persist authentication credentials across Mail Intake components.
+
+The credential persistence layer is intentionally decoupled from
+authentication logic. Authentication providers are responsible for
+credential acquisition, validation, and refresh, while implementations
+within this package are responsible solely for storage and retrieval.
+
+The package provides:
+- A generic ``CredentialStore`` abstraction defining the persistence contract
+- Local filesystem–based storage for development and single-node use
+- Distributed, Redis-backed storage for production and scaled deployments
+
+Credential lifecycle management, interpretation, and security policy
+decisions remain the responsibility of authentication providers.
+"""
+
 from mail_intake.credentials.store import CredentialStore
+from mail_intake.credentials.pickle import PickleCredentialStore
+from mail_intake.credentials.redis import RedisCredentialStore
 
-from google.oauth2.credentials import Credentials
-
-GoogleCredentialStore = CredentialStore[Credentials]
-
-"""
+__all__ = [
+    "CredentialStore",
+    "PickleCredentialStore",
+    "RedisCredentialStore",
+]