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.

README.md

@@ -8,6 +8,8 @@ the 1Password CLI (`op`) from `op://API Keys/openrouter-mai-transcribe-2/credent
 ./transcribe recording.m4a
 ./transcribe video.mp4 > transcript.txt
 ./transcribe --srt video.mp4 > subtitles.srt
+./transcribe --no-diarization recording.m4a > transcript.txt
+./transcribe --srt --no-diarization video.mp4 > subtitles.srt
 ./transcribe --chunk-minutes 10 too_long.webm > transcript.txt
 ```
 
@@ -18,10 +20,23 @@ and sends it to OpenRouter's `microsoft/mai-transcribe-2` model. Temporary
 files are removed on exit. The transcript goes to stdout; progress and errors
 go to stderr. Output is emitted only after all requests and conversion succeed.
 
-Long recordings are automatically split into uploads of at most **30 minutes**,
-including overlap and MP3 padding. Use `--chunk-minutes N` (an integer from 1 to
-30) to lower the limit. At 64 kbps, a 30-minute upload is about 14.4 MB of MP3
-or 19.2 MB of base64. This is a local default, not a guaranteed provider limit:
+Speaker diarization is **enabled by default** through
+`provider.options.azure.diarization.enabled`, following
+[OpenRouter's speaker diarization API](https://openrouter.ai/docs/guides/overview/multimodal/stt#verbose-transcripts-timestamps-and-speakers).
+Use `--no-diarization` to explicitly disable it for every request, with either
+plain text or `--srt` output. SRT uses returned speaker labels to break cues at
+speaker changes; neither output format prints speaker names or IDs.
+
+Long recordings are automatically split into uploads of at most **10 minutes**
+with diarization, or **30 minutes** with `--no-diarization`, including overlap
+and MP3 padding. These defaults account for
+[Microsoft's documented diarization failures around 15 minutes and longer](https://learn.microsoft.com/en-us/azure/ai-services/speech-service/mai-transcribe).
+Use `--chunk-minutes N` (an integer from 1 to 30) to override the limit in either
+mode. Larger chunks with diarization may fail with a timeout or
+`diarization_unavailable`; reduce the chunk size or use `--no-diarization`.
+At 64 kbps, a 10-minute upload is about 4.8 MB of MP3 or 6.4 MB of base64;
+a 30-minute upload is about 14.4 MB of MP3 or 19.2 MB of base64.
+These are local defaults, not guaranteed provider limits:
 [OpenRouter documents a 60-second processing timeout](https://openrouter.ai/docs/guides/overview/multimodal/stt),
 so reduce the chunk size if requests still time out.