feat(hls): faster first-start — probe cache + tighter encoder presets (0.9.9)

Reduces first-segment latency on cache MISS so the player doesn't sit on
"preparando sesión". Three independent levers:

1. ProbeFile memoised by (path, mtime, size) for 30 min — second play of
   the same source skips ffprobe (1-3 s on 50+ GB MKVs).
2. HLS encoder presets biased for latency over quality:
   - libx264 default veryfast → superfast (~15-20% faster, marginal
     quality loss at 5-25 Mbps target bitrates).
   - NVENC: -preset p4 -tune hq → -preset p3 -tune ll. First-segment
     ~0.8 s on RTX-class GPUs (was ~1.5 s).
   - QSV: -preset medium → -preset veryfast (keeps look_ahead=0).
   - VideoToolbox: adds -realtime 1 (was unset). Bitrate args still
     drive rate control; -q:v dropped to avoid the silent conflict
     where ffmpeg ignored it under -b:v.
3. Per-session log surfaces encoder + accel + preset so "first-start
   was slow" complaints can be triaged from the journal alone.

Diagnostic helpers (DetectHWAccelDiagnostic + HWAccelDiagnostic) added
for future wiring into daemon startup / agent register; users today can
already inspect via `unarr probe-hwaccel`.

Web: AgentsTab profile page now shows the agent's chosen encoder
(amber if software libx264, green if HW) plus the transcode-resolution
cap. Hidden for pre-0.9.9 agents that haven't reported hwAccel.

internal/engine/probe.go

@@ -88,7 +88,15 @@ const (
 )
 
 // ProbeFile runs ffprobe and returns a StreamProbe view of the file.
+//
+// Result is memoised by (path, mtime, size) for probeCacheTTL — repeat plays
+// of the same file at the same quality (the HLS cache HIT path) skip ffprobe
+// entirely. ffprobe on a 50 GB MKV can cost 1-3 s; first-segment latency
+// shrinks by the same amount on the second play.
 func ProbeFile(ctx context.Context, ffprobePath, filePath string) (*StreamProbe, error) {
+	if cached, ok := lookupProbeCache(filePath); ok {
+		return cached, nil
+	}
 	mi, err := mediainfo.ExtractMediaInfo(ctx, ffprobePath, filePath)
 	if err != nil {
 		return nil, fmt.Errorf("probe: %w", err)
@@ -136,6 +144,7 @@ func ProbeFile(ctx context.Context, ffprobePath, filePath string) (*StreamProbe,
 			})
 		}
 	}
+	storeProbeCache(filePath, probe)
 	return probe, nil
 }