fix: backup validation display and debug logging

- Add 4-branch template guard to handle zero-value DumpValidation
  (shows "–" instead of misleading "Hiba" when validation never ran)
- Add debug logging to ValidateDump for all code paths
- Add cross-check re-validation in RefreshCache to heal stale
  lastDBDump validation state from disk

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/backup/dbdump.go

@@ -231,6 +231,7 @@ func DumpOne(ctx context.Context, db DiscoveredDB, dumpDir string, logger *log.L
 
 // ValidateDump checks a SQL dump file for basic structural integrity.
 func ValidateDump(filePath string, dbType DBType) DumpValidation {
+	log.Printf("[DEBUG] ValidateDump: %s (type=%s)", filePath, dbType)
 	stat, err := os.Stat(filePath)
 	if err != nil {
 		return DumpValidation{Error: fmt.Sprintf("stat failed: %v", err)}
@@ -243,12 +244,14 @@ func ValidateDump(filePath string, dbType DBType) DumpValidation {
 
 	if stat.Size() < 100 {
 		v.Error = "dump file too small (< 100 bytes)"
+		log.Printf("[WARN] ValidateDump FAIL: %s — %s", filePath, v.Error)
 		return v
 	}
 
 	data, err := os.ReadFile(filePath)
 	if err != nil {
 		v.Error = fmt.Sprintf("read failed: %v", err)
+		log.Printf("[WARN] ValidateDump FAIL: %s — %s", filePath, v.Error)
 		return v
 	}
 
@@ -295,15 +298,18 @@ func ValidateDump(filePath string, dbType DBType) DumpValidation {
 		case DBTypePostgres:
 			v.Error = "PostgreSQL dump missing comment header"
 		}
+		log.Printf("[WARN] ValidateDump FAIL: %s — %s", filePath, v.Error)
 		return v
 	}
 
 	if tableCount == 0 {
 		v.Error = "no CREATE TABLE statements found"
+		log.Printf("[WARN] ValidateDump FAIL: %s — %s (header was found, scanned %d lines)", filePath, v.Error, len(strings.Split(content, "\n")))
 		return v
 	}
 
 	v.Valid = true
+	log.Printf("[DEBUG] ValidateDump OK: %s — %d tables, header found", filePath, tableCount)
 	return v
 }