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>

utils/sdr/__init__.py

@@ -26,7 +26,7 @@ from __future__ import annotations
 from typing import Optional
 
 from .base import CommandBuilder, SDRCapabilities, SDRDevice, SDRType
-from .detection import detect_all_devices, probe_rtlsdr_device
+from .detection import detect_all_devices, invalidate_device_cache, probe_rtlsdr_device
 from .rtlsdr import RTLSDRCommandBuilder
 from .limesdr import LimeSDRCommandBuilder
 from .hackrf import HackRFCommandBuilder
@@ -231,4 +231,5 @@ __all__ = [
     'get_capabilities_for_type',
     # Device probing
     'probe_rtlsdr_device',
+    'invalidate_device_cache',
 ]