refactor: stop passing VideoStore an audio decoder it never used

VideoStore took an AVCodecContext for the input audio and ignored it: the
member was initialised to nullptr and open() builds its own from
audio_in_stream's codecpar, with its own codec lookup, time_base and
avcodec_open2, and frees it in the destructor.

So the parameter described a relationship that did not exist, and following it
backwards -- through Monitor::GetAudioCodecContext() to the camera -- suggested
the camera's audio decoder was feeding recording. It was not.

Drop the parameter and say beside the member where the context actually comes
from.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Y6FieTwEXuLhhR4e2yiax

src/zm_event.cpp

@@ -862,7 +862,6 @@ void Event::Run() {
       monitor->GetVideoStream(),
       monitor->GetVideoCodecContext(),
       ( monitor->RecordAudio() ? monitor->GetAudioStream() : nullptr ),
-      ( monitor->RecordAudio() ? monitor->GetAudioCodecContext() : nullptr ),
       monitor );
 
     if (!videoStore->open()) {

src/zm_videostore.cpp

@@ -43,7 +43,6 @@ VideoStore::VideoStore(
   AVStream *p_video_in_stream,
   AVCodecContext *p_video_in_ctx,
   AVStream *p_audio_in_stream,
-  AVCodecContext *p_audio_in_ctx,
   Monitor *p_monitor
 ) :
   chosen_codec_data(nullptr),
@@ -57,6 +56,8 @@ VideoStore::VideoStore(
   video_in_stream(p_video_in_stream),
   audio_in_stream(p_audio_in_stream),
   audio_in_codec(nullptr),
+  // Opened in open() from audio_in_stream's codecpar, not handed in. The
+  // caller used to pass a context as well and this ignored it.
   audio_in_ctx(nullptr),
   audio_out_codec(nullptr),
   audio_out_ctx(nullptr),

src/zm_videostore.h

@@ -138,7 +138,6 @@ class VideoStore {
     AVStream *video_in_stream,
     AVCodecContext  *video_in_ctx,
     AVStream *audio_in_stream,
-    AVCodecContext  *audio_in_ctx,
     Monitor * p_monitor);
   ~VideoStore();
   bool  open();