feat(doorbird): add continuous audio for live view and HKSV recordings

[mrsvan]

Summary

• Add a Continuous Audio setting (enabled by default) that keeps a persistent connection to the Doorbird microphone (audio-receive.cgi)
• When disabled, the original silence generator behavior is preserved
• Auto-reconnects on connection drop via FFmpeg reconnect flags and process restart

Problem

The Doorbird D101S (and other models) does not include audio in its RTSP video stream. Audio and video use separate endpoints:

• Video: RTSP at /mpeg/media.amp (H.264 only, no audio track)
• Audio: HTTP at /bha-api/audio-receive.cgi (pcm_mulaw, 8kHz, mono)

The plugin currently feeds silence (anullsrc) into the audio track and only connects to the real microphone during two-way intercom. This means:

• HomeKit live view has no audio
• HKSV recordings have no audio
• Audio only works when the intercom/talk button is pressed

This has been reported in #934 and #996.

Solution

Replace the silence generator with a continuous connection to audio-receive.cgi so the Doorbird's microphone audio flows at all times. The change is in startSilenceGenerator() which conditionally uses either the real audio endpoint or the original anullsrc based on the new setting.

Key design decisions:

• Opt-in setting (defaults to enabled) for backward compatibility
• FFmpeg -reconnect flags handle HTTP stream drops gracefully
• Auto-restart on exit with 2-second delay if the ffmpeg process exits unexpectedly
• No changes to intercom flow — the existing doorbellAudioActive guard ensures intercom audio takes priority

Test plan

• Enable "Continuous Audio" (default) — verify audio in HomeKit live view without pressing talk
• Trigger motion event — verify HKSV recording has audio in Apple Home timeline
• Press talk button — verify two-way intercom still works correctly
• Disable "Continuous Audio" — verify silence is restored (original behavior)
• Kill the Doorbird network temporarily — verify ffmpeg reconnects automatically

Fixes #934, fixes #996

[koush]

What's the implications of having the intercom always on? Does this change the audio provide to use echo cancellation and stuff like other 2 way systems?