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

internal/db/db_test.go

@@ -14,14 +14,18 @@ func TestDatabaseOperations(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tempDir)
+	defer func() {
+		_ = os.RemoveAll(tempDir)
+	}()
 
 	dbPath := filepath.Join(tempDir, "test.db")
 	database, err := InitDB(dbPath)
 	if err != nil {
 		t.Fatalf("InitDB failed: %v", err)
 	}
-	defer database.Close()
+	defer func() {
+		_ = database.Close()
+	}()
 
 	// 1. Test User
 	u, err := database.CreateUser("admin", "$2a$10$dummyhashvalue")