fix(api): use proper Pydantic models in vendor themes/domains

vendor_themes.py:
- Return ThemePresetListResponse instead of raw dict (API-001 fix)
- Add ThemePresetResponse response_model to apply_theme_preset
- Add ThemeDeleteResponse response_model to delete endpoint

vendor_domain.py:
- Remove _get_vendor_by_id helper with direct DB query
- Use vendor_service.get_vendor_by_id() instead (API-002 fix)

models/schema/vendor_theme.py:
- Add ThemeDeleteResponse model for delete endpoint response

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

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

models/schema/vendor_theme.py

@@ -100,3 +100,9 @@ class ThemePresetListResponse(BaseModel):
     """List of available theme presets."""
 
     presets: list[ThemePresetPreview] = Field(..., description="Available presets")
+
+
+class ThemeDeleteResponse(BaseModel):
+    """Response after deleting a theme."""
+
+    message: str = Field(..., description="Success message")