perf(stream): run the subtitle/thumbnail prewarm at idle I/O priority

The prewarm's single big read (a ~14 min sequential pass over a 60GB remux to
demux subtitles) shares the same disk/NFS as live streaming. Lower the prewarm
ffmpeg processes to the Linux IDLE I/O class (ioprio_set) so that background
read yields bandwidth to a user who's actually watching — the prewarm slows
down under contention instead of starving playback, and runs full speed when the
disk is idle.

Applied only to the prewarm-only extractors (ExtractSubtitlesVTTMulti,
ExtractThumbnailJPEG) via Start → setIdleIOPriority(pid) → Wait; the on-demand
/sub + /thumbnail handlers keep normal priority (a user is waiting on those).
Linux-only syscall behind a build tag; a no-op stub elsewhere. Best-effort —
errors ignored, never required for correctness.

Verified: the prewarm ffmpeg shows 'idle' under ionice -p; on-demand stays normal.

internal/library/mediainfo/ioprio_linux.go

@@ -0,0 +1,25 @@
+//go:build linux
+
+package mediainfo
+
+import "syscall"
+
+// Linux I/O priority (ioprio) constants. The 16-bit ioprio value packs a class
+// in the top 3 bits (shift 13) and a class-data nibble below it; the IDLE class
+// takes no data.
+const (
+	ioprioWhoProcess = 1 // IOPRIO_WHO_PROCESS
+	ioprioClassIdle  = 3 // IOPRIO_CLASS_IDLE
+	ioprioClassShift = 13
+)
+
+// setIdleIOPriority best-effort lowers a process's I/O scheduling class to IDLE,
+// so a long background read (the subtitle prewarm of a huge remux — a single
+// ~14 min sequential read of a 60GB file over NFS) yields disk/NFS bandwidth to
+// foreground work like live streaming. Linux-only; on kernels or filesystems
+// that don't honor ioprio this simply has no effect. Errors are intentionally
+// ignored — it's an optimization, never required for correctness.
+func setIdleIOPriority(pid int) {
+	ioprio := ioprioClassIdle << ioprioClassShift // IDLE class, data 0
+	_, _, _ = syscall.Syscall(syscall.SYS_IOPRIO_SET, uintptr(ioprioWhoProcess), uintptr(pid), uintptr(ioprio))
+}