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/vendor/features.py

@@ -16,12 +16,13 @@ Endpoints:
 
 import logging
 
-from fastapi import APIRouter, Depends, HTTPException, Query
+from fastapi import APIRouter, Depends, Query
 from pydantic import BaseModel
 from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
+from app.exceptions import FeatureNotFoundError
 from app.services.feature_service import feature_service
 from models.database.user import User
 
@@ -99,6 +100,13 @@ class FeatureGroupedResponse(BaseModel):
     total_count: int
 
 
+class FeatureCheckResponse(BaseModel):
+    """Quick feature availability check response."""
+
+    has_feature: bool
+    feature_code: str
+
+
 # ============================================================================
 # Endpoints
 # ============================================================================
@@ -285,7 +293,7 @@ def get_feature_detail(
     # Get feature
     feature = feature_service.get_feature_by_code(db, feature_code)
     if not feature:
-        raise HTTPException(status_code=404, detail=f"Feature '{feature_code}' not found")
+        raise FeatureNotFoundError(feature_code)
 
     # Check availability
     is_available = feature_service.has_feature(db, vendor_id, feature_code)
@@ -317,7 +325,7 @@ def get_feature_detail(
     )
 
 
-@router.get("/check/{feature_code}")
+@router.get("/check/{feature_code}", response_model=FeatureCheckResponse)
 def check_feature(
     feature_code: str,
     current_user: User = Depends(get_current_vendor_api),
@@ -332,9 +340,9 @@ def check_feature(
         feature_code: The feature code
 
     Returns:
-        {"has_feature": true/false}
+        has_feature and feature_code
     """
     vendor_id = current_user.token_vendor_id
     has_feature = feature_service.has_feature(db, vendor_id, feature_code)
 
-    return {"has_feature": has_feature, "feature_code": feature_code}
+    return FeatureCheckResponse(has_feature=has_feature, feature_code=feature_code)