docs(mail_intake): add comprehensive docstrings across ingestion, adapters, auth, and parsing layers

- docs(mail_intake/__init__.py): document module-based public API and usage patterns
- docs(mail_intake/ingestion/reader.py): document high-level ingestion orchestration
- docs(mail_intake/adapters/base.py): document adapter contract for mail providers
- docs(mail_intake/adapters/gmail.py): document Gmail adapter implementation and constraints
- docs(mail_intake/auth/base.py): document authentication provider contract
- docs(mail_intake/auth/google.py): document Google OAuth authentication provider
- docs(mail_intake/models/message.py): document canonical email message model
- docs(mail_intake/models/thread.py): document canonical email thread model
- docs(mail_intake/parsers/body.py): document message body extraction logic
- docs(mail_intake/parsers/headers.py): document message header normalization utilities
- docs(mail_intake/parsers/subject.py): document subject normalization utilities
- docs(mail_intake/config.py): document global configuration model
- docs(mail_intake/exceptions.py): document library exception hierarchy

mail_intake/auth/__init__.py

@@ -0,0 +1,26 @@
+"""
+Authentication provider implementations for Mail Intake.
+
+This package defines the **authentication layer** used by mail adapters
+to obtain provider-specific credentials.
+
+It exposes:
+- A stable, provider-agnostic authentication contract
+- Concrete authentication providers for supported platforms
+
+Authentication providers:
+- Are responsible for credential acquisition and lifecycle management
+- Are intentionally decoupled from adapter logic
+- May be extended by users to support additional providers
+
+Consumers should depend on the abstract interface and use concrete
+implementations only where explicitly required.
+"""
+
+from .base import MailIntakeAuthProvider
+from .google import MailIntakeGoogleAuth
+
+__all__ = [
+    "MailIntakeAuthProvider",
+    "MailIntakeGoogleAuth",
+]