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/MusicDetail.tsx

@@ -216,7 +216,7 @@ export const MusicDetail = () => {
       >
         <div className="aspect-square w-full overflow-hidden rounded-md bg-muted">
           <img
-            src={coverUrl(r, token)}
+            src={coverUrl(r, token, manifest[r]?.v)}
             alt=""
             loading="lazy"
             className="h-full w-full object-cover"
@@ -282,7 +282,7 @@ export const MusicDetail = () => {
           <div className="flex items-end gap-5">
             <div className="h-40 w-40 shrink-0 overflow-hidden rounded-lg bg-muted shadow-lg">
               <img
-                src={coverUrl(rel, token)}
+                src={coverUrl(rel, token, manifest[rel]?.v)}
                 alt=""
                 className="h-full w-full object-cover"
                 onError={(e) => {