Fix Bug #12: Bindestrich im Titel wird jetzt erlaubt

- Validierungsregel in AiResponseValidator erweitert um Bindestrich (-)
- Test angepasst: validate_titleWithHyphen jetzt als Valid-Case akzeptiert
- Dokumentation (fachliche Anforderungen, CLAUDE.md) aktualisiert
- mvn clean verify erfolgreich durchgelaufen

pdf-umbenenner-application/src/main/java/de/gecheckt/pdf/umbenenner/application/service/AiResponseValidator.java

@@ -102,7 +102,7 @@ public final class AiResponseValidator {
 
         if (!isAllowedTitleCharacters(title)) {
             return AiValidationResult.invalid(
-                    "Title contains disallowed characters (only letters, digits, and spaces are permitted): '"
+                    "Title contains disallowed characters (only letters, digits, spaces, and hyphens are permitted): '"
                             + title + "'",
                     AiErrorClassification.FUNCTIONAL);
         }
@@ -142,14 +142,14 @@ public final class AiResponseValidator {
     // -------------------------------------------------------------------------
 
     /**
-     * Returns {@code true} if every character in the title is a letter, digit, or space.
+     * Returns {@code true} if every character in the title is a letter, digit, space, or hyphen.
      * <p>
      * Permits Unicode letters including German Umlauts (ä, ö, ü, Ä, Ö, Ü) and ß.
      */
     private static boolean isAllowedTitleCharacters(String title) {
         for (int i = 0; i < title.length(); i++) {
             char c = title.charAt(i);
-            if (!Character.isLetter(c) && !Character.isDigit(c) && c != ' ') {
+            if (!Character.isLetter(c) && !Character.isDigit(c) && c != ' ' && c != '-') {
                 return false;
             }
         }