Implements GmailConnector — the first concrete SourceConnector.
Wraps existing GmailClient + low-level Gmail API service for metadata-only
fetch, trash archive, incremental history polling, and Pub/Sub watch setup.
Adds GMAIL_PUBSUB_TOPIC setting (empty string default for dev).
Adds 3 passing unit tests (mocked API, no real credentials required).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
