feat: implement Gmail API mail source with OAuth2, ingestion tracking, and UI

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/0c63851e-a69a-4a76-8dd8-25618e825b8c

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/models/mail_source.py

@@ -15,7 +15,7 @@ class MailSource(Base):
 
     - ``IMAP``       – standard IMAP4 (over SSL/TLS or STARTTLS)
     - ``POP3``       – POP3 inbox (stub for future implementation)
-    - ``GMAIL_API``  – Gmail API with OAuth 2.0 (stub for future implementation)
+    - ``GMAIL_API``  – Gmail API with OAuth 2.0
     """
 
     __tablename__ = "mail_sources"
@@ -38,6 +38,17 @@ class MailSource(Base):
     use_ssl = Column(Boolean, default=True)
     folder = Column(String, default="INBOX")
 
+    # Gmail API OAuth2 credentials (used by GMAIL_API method)
+    # NOTE: tokens stored in plaintext – encrypt at the app layer in production.
+    gmail_client_id = Column(String, nullable=True)
+    gmail_client_secret = Column(Text, nullable=True)
+    gmail_access_token = Column(Text, nullable=True)
+    gmail_refresh_token = Column(Text, nullable=True)
+    # Email address of the authorised Gmail account
+    gmail_email = Column(String, nullable=True)
+    # JSON-encoded list of Gmail message IDs that have already been ingested
+    gmail_ingested_ids = Column(Text, nullable=True, default="[]")
+
     # Polling behaviour
     polling_interval = Column(Integer, default=60)  # minutes