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/core/config.py

@@ -46,6 +46,10 @@ class Settings(BaseSettings):
     GOOGLE_CLIENT_SECRET: Optional[str] = None
     GOOGLE_REDIRECT_URI: str = "http://localhost:3000/auth/callback/google"
     
+    # Gmail API (for direct email injection)
+    GMAIL_API_ENABLED: bool = True
+    GMAIL_INJECT_LABEL_IDS: List[str] = ["INBOX"]
+    
     # CORS
     CORS_ORIGINS: List[str] = ["http://localhost:3000", "http://localhost:8000"]