Add customizable recording beep sound feature

Backend:
- Add beep_sound and beep_enabled configuration options
- Generate default 1000Hz beep (0.5s) on first run
- Add get/set methods for beep sound and enabled state
- Play beep after greeting, before recording starts
- Add /set_beep_sound and /api/beep_enabled endpoints

Frontend:
- Add "Recording Beep" toggle checkbox in settings
- Add "📣 Set Beep" button for each greeting sound
- Show beep status indicator on sounds
- Real-time enable/disable with visual feedback
- Template v1.4.0

Configuration:
- beep_sound: WAV filename for beep (default: "beep.wav")
- beep_enabled: true/false to enable/disable beep

This gives guests a clear audio cue that recording has started.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

config.example.json

@@ -35,6 +35,8 @@
   "system": {
     "active_greeting": "dialtone.wav",
     "extra_button_sound": "button_sound.wav",
+    "beep_sound": "beep.wav",
+    "beep_enabled": true,
     "greeting_delay_seconds": 0,
     "volume": 70
   }