feat(scouts): add GmailConnector

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>

app/config/settings.py

@@ -58,6 +58,11 @@ class Settings(BaseSettings):
     # Prod: https://api.adiuvai.com/api/v1/auth/oauth/google/web-callback
     OAUTH_REDIRECT_URI: str = "http://localhost:8000/api/v1/auth/oauth/google/web-callback"
 
+    # Gmail Pub/Sub topic for push notifications.
+    # Full resource name, e.g. "projects/my-project/topics/gmail-push".
+    # Leave empty in dev — setup_watch will skip registration gracefully.
+    GMAIL_PUBSUB_TOPIC: str = ""
+
     # Fernet key (URL-safe base64, 32-byte key) for at-rest encryption of OAuth
     # tokens stored in cloud_agent_configs.oauth_token_encrypted.
     # Generate with: from cryptography.fernet import Fernet; Fernet.generate_key()