Add Gmail API injection, provider presets, delivery method, and frontend wizard

- Add gmail_service.py with Gmail API users.messages.insert() for direct email injection
- Add DeliveryMethod enum and GmailCredential model to database models
- Add delivery_method field to MailAccount schema and model
- Create providers.py endpoint with 12 provider presets (Gmail, GMX, WEB.DE, Outlook, Yahoo, AOL, T-Online, 1&1/IONOS, Freenet, Posteo, mail.de, iCloud)
- Expand MailServerAutoDetect with 30+ domain mappings
- Update Celery tasks to prefer Gmail API injection, with SMTP fallback
- Add ProviderWizard.tsx frontend component for quick provider setup
- Update AddMailAccountModal.tsx with wizard integration
- Add Google API Python client libraries to requirements.txt
- Add Gmail API config settings
- Add unit tests for Gmail service and provider presets

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/de3ef930-a980-4958-8a9d-a2c802918e81

backend/requirements.txt

@@ -26,6 +26,12 @@ aiohttp==3.13.3  # Updated: Fixed zip bomb, DoS, and directory traversal vulnera
 aioimaplib==1.0.1
 email-validator==2.1.0.post1
 
+# Gmail API (for direct email injection)
+google-api-python-client==2.193.0
+google-auth==2.49.1
+google-auth-oauthlib==1.2.0
+google-auth-httplib2==0.2.0
+
 # Job Queue & Cache
 celery==5.3.6
 redis==5.0.1