chore: Refactor test setup for Gin framework

- Removed redundant `gin.SetMode(gin.TestMode)` calls from individual test files.
- Introduced a centralized `TestMain` function in `testmain_test.go` to set the Gin mode for all tests.
- Ensured consistent test environment setup across various handler test files.

backend/internal/api/handlers/coverage_quick_test.go

@@ -33,7 +33,6 @@ func createValidSQLiteDB(t *testing.T, dbPath string) error {
 // Use a real BackupService, but point it at tmpDir for isolation
 
 func TestBackupHandlerQuick(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 	tmpDir := t.TempDir()
 	// Create a valid SQLite database for backup operations
 	dbPath := filepath.Join(tmpDir, "db.sqlite")