feat(api): implement inventory API endpoints

Add REST API endpoints for character inventory management:
- GET /api/v1/characters/<id>/inventory - Get inventory and equipped items
- POST /api/v1/characters/<id>/inventory/equip - Equip item to slot
- POST /api/v1/characters/<id>/inventory/unequip - Unequip from slot
- POST /api/v1/characters/<id>/inventory/use - Use consumable item
- DELETE /api/v1/characters/<id>/inventory/<item_id> - Drop item

All endpoints include:
- Authentication via @require_auth decorator
- Ownership validation through CharacterService
- Comprehensive error handling with proper HTTP status codes
- Full logging for debugging

Includes 25 integration tests covering authentication requirements,
URL patterns, and response formats.

Task 2.4 of Phase 4 Combat Implementation complete.

api/app/__init__.py

@@ -174,6 +174,11 @@ def register_blueprints(app: Flask) -> None:
     app.register_blueprint(combat_bp)
     logger.info("Combat API blueprint registered")
 
+    # Import and register Inventory API blueprint
+    from app.api.inventory import inventory_bp
+    app.register_blueprint(inventory_bp)
+    logger.info("Inventory API blueprint registered")
+
     # TODO: Register additional blueprints as they are created
     # from app.api import marketplace, shop
     # app.register_blueprint(marketplace.bp, url_prefix='/api/v1/marketplace')