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

radio.py

@@ -196,7 +196,7 @@ class Radio:
         def attempt(name, fn):
             try:
                 fn(); steps[name] = True
-            except (CIVError, ValueError, OSError) as e:
+            except (CIVError, ValueError, TypeError, OSError) as e:
                 steps[name] = False; errors[name] = str(e)
 
         attempt("freq", lambda: self.set_frequency(int(round(rec["output_mhz"] * 1e6))))