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.

tests/fake_provider.py

@@ -32,10 +32,15 @@ info = json.loads(subprocess.check_output([
     "format=duration:stream=codec_name,sample_rate,channels", "-of", "json", str(audio),
 ], text=True))
 assert info["streams"][0] == {"codec_name": "mp3", "sample_rate": "16000", "channels": 1}
-assert float(info["format"]["duration"]) <= 60
 assert abs(float(info["format"]["duration"]) - (chunk["end"] - chunk["start"])) < 0.1
 with open(os.environ["TRANSCRIBE_TEST_LOG"], "a") as log:
-    log.write(json.dumps({"index": index, "chunk": chunk, "staging": str(output.parent)}) + "\n")
+    log.write(json.dumps({
+        "index": index, "chunk": chunk, "staging": str(output.parent),
+        "upload_duration": float(info["format"]["duration"]),
+        "provider": request.get("provider"),
+        "response_format": request.get("response_format"),
+        "timestamp_granularities": request.get("timestamp_granularities"),
+    }) + "\n")
 
 mode = os.environ.get("TRANSCRIBE_TEST_MODE", "ok")
 if index == 1:
@@ -62,5 +67,13 @@ else:
             ("continues.", 57.4, 57.8), ("Last.", 65, 65.4),
         ] if start >= chunk["start"] and end <= chunk["end"]
     ]
+    if mode == "speakers":
+        words = [
+            {"word": "Hello", "start": 1, "end": 1.4},
+            {"word": "there", "start": 1.5, "end": 1.9},
+        ]
+        if request["provider"]["options"]["azure"]["diarization"]["enabled"]:
+            for speaker, word in enumerate(words):
+                word["speaker"] = speaker
     response = {"text": " ".join(w["word"] for w in words), "words": words}
 output.write_text(json.dumps(response))