Decouple Gmail permissions from Google Sign-In OAuth flow

The Google Sign-In flow now only requests basic profile scopes (openid,
email, profile) instead of also requesting Gmail API scopes. Users can
grant Gmail access separately via the "Connect Gmail" button in Settings.

- Remove GMAIL_SCOPES from GOOGLE_LOGIN_SCOPES in auth.py
- Remove Gmail credential auto-provisioning from google_oauth endpoint
- Simplify /auth/google/authorize-url (no offline access or forced consent)
- Clean up auth_service.py OAuth registration to only use login scopes
- Remove unused imports (GmailCredential, GmailService, encrypt_credential, etc.)

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/91d2db11-811f-46f5-ac89-29952ab358b5

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

backend/app/services/auth_service.py

@@ -25,9 +25,7 @@ class OAuthService:
     def _register_google(self):
         """Register Google OAuth2 provider"""
         if settings.GOOGLE_CLIENT_ID and settings.GOOGLE_CLIENT_SECRET:
-            from app.services.gmail_service import GMAIL_SCOPES
-
-            scope = " ".join(["openid", "email", "profile", *GMAIL_SCOPES])
+            scope = "openid email profile"
             self.oauth.register(
                 name="google",
                 client_id=settings.GOOGLE_CLIENT_ID,