feat: enable diarization by default with an opt-out

Request speaker diarization through OpenRouter's Azure provider options for
all transcription uploads. Add --no-diarization to explicitly disable it for
both plain text and SRT output; returned speaker labels let existing SRT
cue generation split at speaker changes.

Default to 10-minute chunks when diarization is enabled because Microsoft
reports failures around 15 minutes and longer. Preserve the 30-minute
default with the opt-out and honor explicit --chunk-minutes values in both
modes. Document the options, duration limits, and output behavior.

Test Plan:
- python3 -B -m unittest -v: all 32 tests passed, including request options,
  default upload limits, opt-out combinations, and SRT speaker boundaries.
- bash -n transcribe: passed.
- ./transcribe --help: verified the new option and chunk defaults.
- git diff --cached --check: passed.
- Provider responses were mocked; no live API request was made.

transcribe

@@ -3,16 +3,19 @@
 set -euo pipefail
 
 srt=false
-chunk_minutes=30
+diarization=true
+chunk_minutes=
 usage() {
-    echo "Usage: ${0##*/} [--srt] [--chunk-minutes 1..30] AUDIO_FILE"
+    echo "Usage: ${0##*/} [--srt] [--no-diarization] [--chunk-minutes 1..30] AUDIO_FILE"
     echo 'Transcribe audio using OPENROUTER_API_KEY; print the text to stdout.'
     echo '  --srt  Print SRT subtitles in short cues using word timestamps instead.'
-    echo '  --chunk-minutes N  Maximum upload duration in minutes (default: 30).'
+    echo '  --no-diarization  Disable speaker diarization (enabled by default).'
+    echo '  --chunk-minutes N  Maximum upload duration in minutes (default: 10; 30 with --no-diarization).'
 }
 while [[ $# -gt 0 ]]; do
     case $1 in
         --srt) srt=true; shift ;;
+        --no-diarization) diarization=false; shift ;;
         --chunk-minutes)
             if [[ ! ${2:-} =~ ^([1-9]|[12][0-9]|30)$ ]]; then
                 echo '--chunk-minutes must be an integer between 1 and 30' >&2
@@ -27,6 +30,10 @@ while [[ $# -gt 0 ]]; do
         *) break ;;
     esac
 done
+if [[ -z $chunk_minutes ]]; then
+    # Azure documents diarization failures for recordings around 15 minutes.
+    if "$diarization"; then chunk_minutes=10; else chunk_minutes=30; fi
+fi
 if [[ $# != 1 ]]; then
     usage >&2
     exit 1
@@ -76,7 +83,9 @@ for ((i = 0; i < count; i++)); do
     # Keep large audio payloads out of shell arguments (which have a size limit).
     base64 <"$tmp/audio.mp3" | tr -d '\n\r' >"$tmp/audio.b64"
     jq -n --rawfile data "$tmp/audio.b64" --argjson timestamps "$timestamps" \
-        '{model: "microsoft/mai-transcribe-2", input_audio: {data: $data, format: "mp3"}}
+        --argjson diarization "$diarization" \
+        '{model: "microsoft/mai-transcribe-2", input_audio: {data: $data, format: "mp3"},
+          provider: {options: {azure: {diarization: {enabled: $diarization}}}}}
          + (if $timestamps then {response_format: "verbose_json", timestamp_granularities: ["segment", "word"]}
             else {} end)' \
         >"$tmp/request.json"