D1: webm remux integration test with no-Cues fixture + idempotency/chain coverage

- Regenerated fixture medarecorder-emu.webm with ffmpeg -live 1 to
  suppress Cues and Duration, emulating browser MediaRecorder output
- Integration test now asserts fixture lacked duration before remux
- Added remuxUpload function call test (real module, not inline ffmpeg)
- Added prepareAudio-chain test: remux -> 16kHz WAV conversion succeeds
- Added idempotency assertion: second remux pass preserves duration
- Added tmp-test-remux to .gitignore

.gitignore

@@ -18,6 +18,10 @@ Thumbs.db
 !.env.example
 !.env.test
 
+# Test temp files
+tmp-test-remux/
+tmp-test-*/
+
 # Vite
 vite.config.js.timestamp-*
 vite.config.ts.timestamp-*