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>

mkdocs.yml

@@ -171,6 +171,7 @@ nav:
       - Stock Management Integration: implementation/stock-management-integration.md
       - Email Templates Architecture: implementation/email-templates-architecture.md
       - Password Reset: implementation/password-reset-implementation.md
+      - Email Settings: implementation/email-settings.md
 
     # --- Testing ---
     - Testing:
@@ -218,6 +219,7 @@ nav:
       - Product Management: guides/product-management.md
       - Inventory Management: guides/inventory-management.md
       - Subscription Tier Management: guides/subscription-tier-management.md
+      - Email Settings: guides/email-settings.md
       - Email Templates: guides/email-templates.md
       - Shop Setup: guides/shop-setup.md
       - CSV Import: guides/csv-import.md