feat(meet-kevin): caption extractor via yt-dlp

- Implement CaptionResult frozen dataclass for structured caption data
- Add parse_srt() to parse SubRip format with flexible timestamp handling
- Add extract_captions() async function using yt-dlp subprocess wrapper
- Prefer manual captions over auto-generated; clean up SRT files after parsing
- Add 16 comprehensive tests covering edge cases (empty input, malformed SRT,
  timestamp variations, language extraction, manual vs auto selection)
- Type-safe implementation with full mypy --strict compliance
- Add sample.srt fixture with 3 segments mentioning NVDA for test reference

tests/fixtures/sample.srt

@@ -0,0 +1,11 @@
+1
+00:00:01,000 --> 00:00:04,500
+Welcome back to Meet Kevin
+
+2
+00:00:04,500 --> 00:00:09,000
+Today we are talking about NVDA and AMD earnings
+
+3
+00:00:09,000 --> 00:00:14,250
+You will want to watch this until the end