Merge Speakers

Merge multiple speakers into a single selected speaker on an audio document.

The merged speakers' segments are relabeled to selected_speaker_id and the merged-away speaker entries are removed from the audio. If the audio is linked to a video, faces matched to a merged-away speaker are unlinked (speaker_id cleared) and any face matched to the surviving speaker has its personality_traits reset to pending — speaker merge is intentionally NOT a face-merge cascade. See module.audios.controller.merge_speakers_in_audio for full semantics, including the symmetric face-merge fix tracked in #439.