feat: expand Gmail OAuth scopes, unify Google callback URL, store Gmail creds on login

- Add gmail.readonly to GMAIL_SCOPES (gmail_service) and GMAIL_API_SCOPES (providers)
  so users().getProfile() no longer returns 403 insufficientPermissions
- Consolidate Gmail scope list: GMAIL_SCOPES in gmail_service.py is the single
  source of truth; auth.py and providers.py now import and spread it
- Add include_granted_scopes=true to both Gmail and login authorize URLs so
  scope additions take effect for previously-connected users
- Add state=gmail_connect to the Gmail authorize URL; the shared /auth/callback
  page routes to gmailApi.saveCallback() when this state is present, otherwise
  falls through to the normal login flow
- Google Sign-In authorize URL now requests all six scopes (openid, email,
  profile + 3 Gmail scopes) with access_type=offline, prompt=consent, and
  include_granted_scopes=true
- POST /auth/google now stores Gmail credentials automatically after sign-in
  (non-fatal: login succeeds even if credential storage fails)
- Settings Connect Gmail button now uses /auth/callback instead of
  /auth/gmail-callback - only ONE redirect URI needed in Google Cloud Console
- URL-encode scope parameter in both authorize URL builders
- Update auth_service._register_google scope to include all Gmail scopes
- Update CHANGELOG.md and docs/TODO.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/d6e2de8b-088d-46f3-8127-137245c3ecdd

backend/app/services/gmail_service.py

@@ -21,6 +21,7 @@ logger = logging.getLogger(__name__)
 GMAIL_SCOPES = [
     "https://www.googleapis.com/auth/gmail.insert",
     "https://www.googleapis.com/auth/gmail.labels",
+    "https://www.googleapis.com/auth/gmail.readonly",
 ]