fix: resolve all golangci-lint errors and integrate linter into CI/CD, Makefile, and check script

cmd/server/main.go

@@ -28,7 +28,9 @@ func main() {
 	if err != nil {
 		log.Fatalf("Fatal: Database initialization failed: %v", err)
 	}
-	defer database.Close()
+	defer func() {
+		_ = database.Close()
+	}()
 	log.Printf("==> Database connected: %s (WAL mode enabled)\n", cfg.DBPath)
 
 	// 2. Initialize Auth Service & Bootstrap Admin