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

server.py

@@ -90,7 +90,10 @@ class Handler(BaseHTTPRequestHandler):
             from urllib.parse import urlparse, parse_qs
             qs = parse_qs(urlparse(self.path).query)
             q = qs.get("q", [""])[0]
-            limit = int(qs.get("limit", ["25"])[0])
+            try:
+                limit = int(qs.get("limit", ["25"])[0])
+            except ValueError:
+                limit = 25
             recs = repeaters.search(repeaters.load(REPEATERS_JSON), q, limit)
             self._json({"ok": True, "repeaters": recs})
         else:
@@ -101,7 +104,7 @@ class Handler(BaseHTTPRequestHandler):
         try:
             body = json.loads(self.rfile.read(length) or b"{}")
             self._json(apply_control(self.path, body))
-        except (CIVError, OSError, ValueError, KeyError) as e:
+        except (CIVError, OSError, ValueError, TypeError, KeyError) as e:
             self._json({"ok": False, "error": str(e)}, 200)