fix: detect and cleanup orphaned import references

Problem: 221 exercises skipped, but only 2 actually in database
Root cause: Failed imports created wiki_import_references entries,
but exercises were never saved (due to earlier bugs)

Duplicate check only verified references existed, not actual data.
Result: All subsequent imports skipped 'already imported' items.

Solution:
- Enhanced duplicate check to verify BOTH reference AND actual entity
- If reference exists but entity is missing → delete orphaned reference
- Item will be re-imported on same run (no manual cleanup needed)

Check logic:
1. Find reference in wiki_import_references
2. Check if exercises/skills/methods table has matching local_id
3. If both exist → skip (true duplicate)
4. If reference exists but entity missing → delete reference + re-import

This auto-heals broken state from previous failed imports.

backend/routers/import_wiki.py

@@ -284,18 +284,37 @@ async def _run_import(
             page_title = member["title"]
             page_id    = member.get("pageid")
 
-            # Duplikat-Check
+            # Duplikat-Check: Prüfe ob bereits importiert UND ob tatsächlich existiert
             if not reimport:
                 with get_db() as conn:
                     cur = get_cursor(conn)
                     cur.execute(
-                        "SELECT id FROM wiki_import_references WHERE wiki_page_title = %s AND content_type = %s",
+                        "SELECT local_id FROM wiki_import_references WHERE wiki_page_title = %s AND content_type = %s",
                         (page_title, import_type)
                     )
-                    if cur.fetchone():
-                        stats["skipped"] += 1
-                        _update_log(log_id, **stats)
-                        continue
+                    ref = cur.fetchone()
+                    if ref:
+                        # Prüfe ob die Übung tatsächlich existiert
+                        if import_type == "exercise":
+                            cur.execute("SELECT id FROM exercises WHERE id = %s", (ref['local_id'],))
+                        elif import_type == "skill":
+                            cur.execute("SELECT id FROM skills WHERE id = %s", (ref['local_id'],))
+                        else:
+                            cur.execute("SELECT id FROM training_methods WHERE id = %s", (ref['local_id'],))
+
+                        if cur.fetchone():
+                            # Referenz UND Übung existieren → Skip
+                            stats["skipped"] += 1
+                            _update_log(log_id, **stats)
+                            continue
+                        else:
+                            # Referenz existiert, aber Übung fehlt → Lösche fehlerhafte Referenz
+                            logger.warning("Orphaned reference found for '%s', re-importing", page_title)
+                            cur.execute(
+                                "DELETE FROM wiki_import_references WHERE wiki_page_title = %s AND content_type = %s",
+                                (page_title, import_type)
+                            )
+                            conn.commit()
 
             # SMW Properties abrufen (mit Retry)
             try: