three bottlenecks in streaming and loading

Measured, not guessed.

ffprobe is gone from the stream path. It ran on the first request for each
file — 50ms warm, 315ms cold on this machine, paid before the first byte, once
per file per sidecar restart — to fill X-Audio-Duration, a header nothing in
the web read and which duplicates the durationSec the indexer already wrote
into meta.json. Removed rather than cached harder: the fastest work is the work
not done, and a second copy of a number is a second thing that can disagree.
BREAKING for any client that read that header; read /meta instead, which every
client already fetches to build its queue.

/stream gains an ETag and Cache-Control. It was the only route with no
validator at all — cover, meta, lyrics and poster have had one all along — so
every seek and every replay re-read the file over the network. mtime+size
identifies the bytes, since a re-encode replaces the file. A Range request
never answers 304: that would answer a question about part of a file with a
statement about all of it, and players handle it badly.

And `?v=` makes covers cacheable rather than merely revalidatable. `no-cache`
means "ask before reusing", so a grid of forty albums cost forty conditional
requests on every load, all answering 304. The manifest already carries each
album's version, so passing it costs the client nothing — and the server then
answers `immutable`, making a repeat visit send nothing at all.

That last one is deliberately conditional on `v` being present, which keeps it
backward compatible: a client that has not been updated gets today's behaviour
and cannot end up holding a stale cover forever.

web/MusicBrowser.tsx

@@ -105,7 +105,10 @@ export const MusicBrowser = () => {
   };
 
   const crumbs = navFolder ? navFolder.slice(MUSIC_ROOT.length + 1).split('/') : [];
-  const coverFor = (childRel: string) => (manifest[childRel]?.cover ? coverUrl(childRel, token) : null);
+  // The manifest already carries each album's version, so passing it costs nothing and turns every
+  // repeat visit's cover request into no request at all.
+  const coverFor = (childRel: string) =>
+    manifest[childRel]?.cover ? coverUrl(childRel, token, manifest[childRel]?.v) : null;
   const shownLibraries = libraries.filter((l) => fuzzyMatch(query, l));
   const shownFolders = folders.filter((f) => fuzzyMatch(query, f));