keep ffprobe as the floor under the meta.json lookup

the previous commit read the duration from meta.json instead of spawning
ffprobe. correct for an indexed album and wrong everywhere else: /stream
serves any home-relative path and never required an index, so a library that
has not been indexed got no header at all. alpha is exactly that case — its
music cache is empty — which is how this was caught rather than shipped.

"always present" replaced with "present if indexed" is the same regression as
removing the header, one layer down.

so both, in order: meta.json when it can answer, ffprobe when it cannot, each
cached. verified on an indexed album (247 from meta, 0.3ms warm) and on the
same file with its index deleted (7.03 from ffprobe, 83ms cold, 0.1ms warm).

MUSIC_API.md

@@ -43,7 +43,9 @@ Byte-range streaming so the player can **seek without downloading the whole file
   client and stays in the contract. New clients should still prefer `durationSec` from `/meta` — it
   arrives with the queue, before the first stream request.
 
-  Absent when the album has not been indexed, exactly as it was absent when ffprobe failed.
+  `meta.json` is the fast path, not the only one: `/stream` serves any home-relative path and does not
+  require indexing, so an unindexed album falls back to ffprobe (cached per file) rather than dropping
+  the header. Absent only if ffprobe is missing or the file is unreadable — as before.
 - **`ETag`** is `mtime-size` in base36. A whole-file request with a matching `If-None-Match` answers
   `304`; a Range request never does, because a 304 would answer a question about part of a file with a
   statement about all of it.