Add web toggle for Bluetooth discoverable window

The board stays hidden by default, so new phones could only pair via an
SSH session. Add 'radxa-audio discoverable on|off' backed by a transient
radxa-bt-pairing unit that auto-approves and trusts devices pairing in
the window, expires on its own, and expose it in the web mixer with a
countdown.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/web_server.py

@@ -48,6 +48,12 @@ def validate_action(data):
             return ['output','bluetooth',addr.upper()]
         raise ValueError('출력 종류가 잘못됐습니다.')
     if action=='scan': return ['scan','--seconds','15']
+    if action=='discoverable':
+        on=data.get('on'); seconds=data.get('seconds')
+        if type(on) is not bool: raise ValueError('검색 허용 값이 잘못됐습니다.')
+        if not on: return ['discoverable','off']
+        if seconds not in (120,300,900) or type(seconds) is not int: raise ValueError('검색 허용 시간: 2, 5, 15분')
+        return ['discoverable','on','--seconds',str(seconds)]
     if action=='pair-output':
         addr=data.get('address')
         if not isinstance(addr,str) or not audio.MAC.fullmatch(addr.upper()): raise ValueError('장비 주소가 잘못됐습니다.')