refactor(core): remove scheduler and simplify app lifecycle

- Delete scheduler.go and scheduler_test.go (no longer needed)
- Simplify App struct by removing Handler interface dependency
- Remove StartScheduler/StopScheduler from app lifecycle
- Update main.go to not pass handler to app constructor
- Remove scheduler mock from app tests, simplify test coverage

cmd/server/main.go

@@ -181,14 +181,14 @@ func main() {
 	}
 
 	// Register all routes and get ebook handler
-	ebookHandler := router.RegisterRoutes(routerConfig)
+	_ = router.RegisterRoutes(routerConfig)
 
 	// ========================================================================
 	// APPLICATION LIFECYCLE MANAGEMENT
 	// ========================================================================
 
 	// Create app with lifecycle management
-	application := app.New(e, ebookHandler)
+	application := app.New(e)
 
 	// ========================================================================
 	// START SERVER (managed by app lifecycle)