fix: harden endpoints against malformed input (final-review findings)

- /api/repeaters: bad ?limit= falls back to 25 instead of crashing the handler
- repeaters.load(): tolerate a corrupted repeaters.json (JSONDecodeError -> [])
- tune_repeater/do_POST: catch TypeError so a null field yields {ok:false} not a reset

repeaters.py

@@ -72,7 +72,7 @@ def load(path):
     try:
         with open(path) as f:
             data = json.load(f)
-    except FileNotFoundError:
+    except (FileNotFoundError, json.JSONDecodeError):
         return []
     return data if isinstance(data, list) else data.get("repeaters", [])