fix: airband start crash when device selector not yet populated

When the /devices fetch hasn't completed or fails, parseInt on an empty
select returns NaN which JSON-serializes to null. The backend then calls
int(None) and raises TypeError. Fix both layers: frontend falls back to
0 on NaN, backend uses `or` defaults so null values don't bypass the
fallback.

Also adds a short TTL cache to detect_all_devices() so multiple
concurrent callers on the same page load don't each spawn blocking
subprocess probes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/listening_post.py

@@ -1311,9 +1311,9 @@ def start_audio() -> Response:
     try:
         frequency = float(data.get('frequency', 0))
         modulation = normalize_modulation(data.get('modulation', 'wfm'))
-        squelch = int(data.get('squelch', 0))
-        gain = int(data.get('gain', 40))
-        device = int(data.get('device', 0))
+        squelch = int(data.get('squelch') or 0)
+        gain = int(data.get('gain') or 40)
+        device = int(data.get('device') or 0)
         sdr_type = str(data.get('sdr_type', 'rtlsdr')).lower()
         request_token_raw = data.get('request_token')
         request_token = int(request_token_raw) if request_token_raw is not None else None