fix: MediaWiki import - increase limit to 500 and add validation

Error: 422 Unprocessable Entity when limit > 100
Root cause: Backend enforced max=100, frontend allowed any value

Backend fix:
- Increased preview limit from 100 to 500 (consistent with execute)
- import_wiki.py line 64: le=100 → le=500

Frontend fix:
- Added Math.min/max validation to both limit inputs
- Preview limit: max 500 with auto-clamp
- Execute limit: max 500 with auto-clamp
- Updated placeholder: 'Kein Limit (max 500)'

Prevents 422 errors from invalid limit values

backend/routers/import_wiki.py

@@ -61,7 +61,7 @@ def require_admin(session: dict = Depends(require_auth)) -> dict:
 async def preview_import(
     category: str = Query(default=CATEGORY_EXERCISES),
     import_type: str = Query(default="exercise"),
-    limit: int = Query(default=10, ge=1, le=100),
+    limit: int = Query(default=10, ge=1, le=500),
     session: dict = Depends(require_admin),
 ):
     """

frontend/src/pages/MediaWikiImportPage.jsx

@@ -202,9 +202,9 @@ export default function MediaWikiImportPage() {
                 <input
                   type="number"
                   value={previewLimit}
-                  onChange={(e) => setPreviewLimit(parseInt(e.target.value) || 10)}
+                  onChange={(e) => setPreviewLimit(Math.min(500, Math.max(1, parseInt(e.target.value) || 10)))}
                   min="1"
-                  max="100"
+                  max="500"
                   style={{
                     width: '100%',
                     padding: '12px',
@@ -368,9 +368,13 @@ export default function MediaWikiImportPage() {
                 <input
                   type="number"
                   value={executeLimit || ''}
-                  onChange={(e) => setExecuteLimit(e.target.value ? parseInt(e.target.value) : null)}
-                  placeholder="Kein Limit"
+                  onChange={(e) => {
+                    const val = e.target.value ? parseInt(e.target.value) : null
+                    setExecuteLimit(val ? Math.min(500, Math.max(1, val)) : null)
+                  }}
+                  placeholder="Kein Limit (max 500)"
                   min="1"
+                  max="500"
                   style={{
                     width: '100%',
                     padding: '12px',