feat: automatically fill missing video transcripts during sync

[swernerx]

I make offcourse sync <course-url> generate missing video transcripts by default, including videos downloaded by earlier runs. Repeating the same command fills in gaps and skips complete transcript files. --no-transcribe opts out, and --dry-run does not transcribe.

I check the actual Markdown and JSON files even when SQLite marks a transcription complete. Missing Markdown is restored from valid JSON without speech recognition; missing or invalid JSON triggers transcription while preserving existing nonempty Markdown. Each sync retries unfinished videos once, regardless of historical attempt counts, and continues after individual video failures. Output files are written atomically.

I also discover existing videos for cached lessons, including numbered additional Josh Comeau videos, without re-extracting those lessons just for transcription. Fully downloaded Skool courses can fill in missing transcripts without opening a browser. --force explicitly regenerates transcripts along with refreshing course content.

Validation:

• I ran pnpm check: formatting, lint, type checking, all 679 tests, and build passed.
• I exercised the built sync CLI with real SQLite state and local files plus a simulated Cuttledoc executable: existing-download backfill, skipping completed transcripts, and restoring deleted Markdown all passed without remote course access.
• I did not run real speech recognition; it requires the native Cuttledoc CLI and backend assets.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 89.79592% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.74%. Comparing base (ecec1b6) to head (2d18648).

Files with missing lines	Patch %	Lines
src/transcription/pipeline.ts	82.14%	5 Missing ⚠️
src/storage/fileSystem.ts	81.25%	3 Missing ⚠️
src/cli/commands/sync.ts	80.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
+ Coverage   48.78%   50.74%   +1.96%     
==========================================
  Files          52       53       +1     
  Lines        5947     6010      +63     
  Branches     1777     1796      +19     
==========================================
+ Hits         2901     3050     +149     
+ Misses       3044     2958      -86     
  Partials        2        2              

Files with missing lines	Coverage Δ
src/cli/commands/syncHighLevel.ts	59.90% <100.00%> (+0.48%)	⬆️
src/cli/commands/syncJoshComeau.ts	95.77% <100.00%> (+0.55%)	⬆️
src/cli/commands/syncLearningSuite.ts	35.75% <100.00%> (+0.25%)	⬆️
src/cli/commands/syncPiccalilli.ts	77.58% <100.00%> (+0.57%)	⬆️
src/cli/syncPipeline.ts	93.27% <100.00%> (+7.56%)	⬆️
src/transcription/cuttledoc.ts	90.00% <100.00%> (ø)
src/transcription/outputs.ts	100.00% <100.00%> (ø)
src/cli/commands/sync.ts	18.18% <80.00%> (+14.10%)	⬆️
src/storage/fileSystem.ts	78.21% <81.25%> (+11.15%)	⬆️
src/transcription/pipeline.ts	90.00% <82.14%> (-5.92%)	⬇️

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