fix: resolve email settings architecture violations and add tests/docs

- Fix API-002 in admin/settings.py: use service layer for DB delete
- Fix API-001/API-003 in vendor/email_settings.py: add Pydantic response
  models, remove HTTPException raises
- Fix SVC-002/SVC-006 in vendor_email_settings_service.py: use domain
  exceptions, change db.commit() to db.flush()
- Add unit tests for VendorEmailSettingsService
- Add integration tests for vendor and admin email settings APIs
- Add user guide (docs/guides/email-settings.md)
- Add developer guide (docs/implementation/email-settings.md)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/settings.py

@@ -610,7 +610,8 @@ def reset_email_settings(
     for key in EMAIL_SETTING_KEYS:
         setting = admin_settings_service.get_setting_by_key(db, key)
         if setting:
-            db.delete(setting)
+            # Use service method for deletion (API-002 compliance)
+            admin_settings_service.delete_setting(db, key, current_admin.id)
             deleted_count += 1
 
     # Log action