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/app/api/v1/endpoints/mail_accounts.py

@@ -64,6 +64,7 @@ async def create_mail_account(
         username=account_in.username,
         encrypted_password=encrypted_password,
         forward_to=account_in.forward_to,
+        delivery_method=account_in.delivery_method,
         is_enabled=account_in.is_enabled,
         check_interval_minutes=account_in.check_interval_minutes,
         max_emails_per_check=account_in.max_emails_per_check,