D1: webm upload remux step for MediaRecorder output

New src/lib/server/remux.ts:
- remuxUpload(inputPath, jobId) — ffmpeg stream copy to fix Cues/duration
- Detects .webm by extension (case-insensitive), pass-through for others
- Wired into pipeline.ts upload branch after saveUploadedFile(),
  before prepareAudio() — YouTube path untouched

Regression tests (all passing, 0 regressions across 158 tests):
- Mock-based unit tests: ffmpeg invocation, non-webm pass-through, error cleanup
- Integration test w/ synthetic opus-in-webm fixture (lower-confidence flag)
  verifies remux produces valid webm with finalized duration

src/lib/server/pipeline.ts

@@ -90,6 +90,9 @@ async function runJob(
 		} else {
 			rawAudioPath = await saveUploadedFile(input.buffer, input.filename, jobId);
 			title = input.filename.replace(/\.[^.]+$/, '');
+			// Remux webm files (MediaRecorder output) to fix Cues/duration
+			const { remuxUpload } = await import('./remux.js');
+			rawAudioPath = await remuxUpload(rawAudioPath, jobId);
 		}
 
 		updateJob({ id: jobId, title });