safe move + sqlite WAL + log indexes + backup with logs

- fix: imap folder names with spaces (RFC3501 quoting in move/create/select)
- fix: move only deletes source after COPY + Message-ID verification in target
- fix: backup endpoint hung on sqlite write locks — enable WAL + busy_timeout
- perf: indexes on filter_logs(created_at, level, account_id+created_at) for
  fast log queries on millions of rows
- feat: optional "logs mit sichern" checkbox in backup export, restore on import
- UI: backup download uses fetch+blob with error display instead of location.href

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routers/yaml_sync.py

@@ -1,4 +1,6 @@
-from fastapi import APIRouter, Depends, UploadFile
+from datetime import datetime
+
+from fastapi import APIRouter, Depends, Query, UploadFile
 from fastapi.responses import PlainTextResponse, Response
 from sqlalchemy.orm import Session
 
@@ -23,12 +25,18 @@ async def yaml_import(file: UploadFile, db: Session = Depends(get_db)):
 
 
 @router.get("/backup")
-def backup_export(db: Session = Depends(get_db)):
-    content = export_backup(db)
+def backup_export(
+    include_logs: bool = Query(default=False),
+    db: Session = Depends(get_db),
+):
+    content = export_backup(db, include_logs=include_logs)
+    suffix = "-mit-logs" if include_logs else ""
+    timestamp = datetime.utcnow().strftime("%Y%m%d-%H%M%S")
+    filename = f"mailfilter-backup{suffix}-{timestamp}.json"
     return Response(
         content=content,
         media_type="application/json",
-        headers={"Content-Disposition": "attachment; filename=mailfilter-backup.json"},
+        headers={"Content-Disposition": f"attachment; filename={filename}"},
     )