test: add service tests and fix architecture violations

- Add comprehensive unit tests for FeatureService (24 tests)
- Add comprehensive unit tests for UsageService (11 tests)
- Fix API-002/API-003 architecture violations in feature/usage APIs
- Move database queries from API layer to service layer
- Create UsageService for usage and limits management
- Create custom exceptions (FeatureNotFoundError, TierNotFoundError)
- Fix ValidationException usage in content_pages.py
- Refactor vendor features API to use proper response models
- All 35 new tests passing

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

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

app/api/v1/admin/content_pages.py

@@ -15,6 +15,7 @@ from pydantic import BaseModel, Field
 from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_api, get_db
+from app.exceptions import ValidationException
 from app.services.content_page_service import content_page_service
 from models.database.user import User
 
@@ -178,11 +179,9 @@ def create_vendor_page(
     Vendor pages override platform defaults for a specific vendor.
     """
     if not page_data.vendor_id:
-        from fastapi import HTTPException
-
-        raise HTTPException(
-            status_code=400,
-            detail="vendor_id is required for vendor pages. Use /platform for platform defaults.",
+        raise ValidationException(
+            message="vendor_id is required for vendor pages. Use /platform for platform defaults.",
+            field="vendor_id",
         )
 
     page = content_page_service.create_page(