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/exceptions/__init__.py

@@ -263,6 +263,13 @@ from .onboarding import (
     OnboardingSyncNotCompleteException,
 )
 
+# Feature management exceptions
+from .feature import (
+    FeatureNotFoundError,
+    InvalidFeatureCodesError,
+    TierNotFoundError,
+)
+
 __all__ = [
     # Base exceptions
     "WizamartException",
@@ -456,4 +463,8 @@ __all__ = [
     "OnboardingCsvUrlRequiredException",
     "OnboardingSyncJobNotFoundException",
     "OnboardingSyncNotCompleteException",
+    # Feature exceptions
+    "FeatureNotFoundError",
+    "TierNotFoundError",
+    "InvalidFeatureCodesError",
 ]