fix(local): remux aac into .m4a so the extracted audio is bit-exact (#149)

Extraction always used Matroska (.mka) because it takes essentially any
audio codec. That is right for most codecs but wrong for AAC: MP4 records
the AAC encoder priming (the ~1000 warm-up samples every AAC encoder emits)
in an edit list, and the edit list does not survive into Matroska. The
remuxed track then decodes ~23 ms longer than the source, with the priming
samples played at the head instead of discarded.

Measured on a 5s 640x480 fixture: source audio decodes to 962980 bytes of
PCM, the .mka to 967440 — 4460 bytes / ~23 ms extra, peaking at -66 dBFS.
Inaudible in practice, but it also puts the track fractionally out of step
with its own reported duration, for no reason.

Pick the container by codec instead: aac -> .m4a (keeps the edit list),
everything else -> .mka as before. If the preferred container refuses the
codec, retry into .mka before falling back to keeping the whole video. AAC
is worth the special case because mp4 / mov / m4v — what people actually
upload — almost always carry it.

Adds the strongest available test of the "lossless" claim: decode the audio
straight out of the source mp4, decode the stored extract, assert the PCM is
byte-for-byte equal. Forcing .mka fails it with exactly the 4460-byte delta.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/music/local-extract-fallback.test.ts

@@ -84,6 +84,7 @@ describe("LocalMusicProvider: source video cannot be deleted after extraction (#
     // Fell back to the original container — the extract was discarded.
     expect(resolved!.url.endsWith(".mp4")).toBe(true);
     expect(existsSync(resolved!.url)).toBe(true);
+    expect(existsSync(resolved!.url.replace(/\.mp4$/, ".m4a"))).toBe(false);
     expect(existsSync(resolved!.url.replace(/\.mp4$/, ".mka"))).toBe(false);
 
     const onDisk = statSync(resolved!.url).size;