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_cache.go

@@ -0,0 +1,96 @@
+package engine
+
+import (
+	"os"
+	"sync"
+	"time"
+)
+
+// probeCacheTTL is how long a cached probe stays usable. The cache key
+// already incorporates mtime + size, so the TTL is a defense against
+// runaway memory growth from stale paths, not a freshness guarantee — a
+// rename + recreate at the same inode (rare) would still be caught by the
+// mtime delta.
+const probeCacheTTL = 30 * time.Minute
+
+type probeCacheEntry struct {
+	probe   *StreamProbe
+	expires time.Time
+}
+
+type probeCacheKey struct {
+	path  string
+	mtime int64 // ModTime().UnixNano()
+	size  int64
+}
+
+var (
+	probeCacheMu sync.RWMutex
+	probeCache   = make(map[probeCacheKey]probeCacheEntry)
+)
+
+// lookupProbeCache returns the cached StreamProbe for the given path if its
+// mtime + size still match the value recorded at insert time, AND the cache
+// entry hasn't expired. Any stat failure / mismatch returns (nil, false) so
+// the caller falls through to a fresh ffprobe run.
+func lookupProbeCache(path string) (*StreamProbe, bool) {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return nil, false
+	}
+	key := probeCacheKey{
+		path:  path,
+		mtime: fi.ModTime().UnixNano(),
+		size:  fi.Size(),
+	}
+	probeCacheMu.RLock()
+	entry, ok := probeCache[key]
+	probeCacheMu.RUnlock()
+	if !ok {
+		return nil, false
+	}
+	if time.Now().After(entry.expires) {
+		probeCacheMu.Lock()
+		delete(probeCache, key)
+		probeCacheMu.Unlock()
+		return nil, false
+	}
+	return entry.probe, true
+}
+
+// storeProbeCache stashes a fresh probe result under the (path, mtime, size)
+// key. A subsequent ffprobe-skipping HIT requires the file to still have the
+// same mtime + size — anything else (re-encoded, renamed+recreated at the
+// same path, truncated) misses and triggers a re-probe.
+func storeProbeCache(path string, probe *StreamProbe) {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return
+	}
+	key := probeCacheKey{
+		path:  path,
+		mtime: fi.ModTime().UnixNano(),
+		size:  fi.Size(),
+	}
+	probeCacheMu.Lock()
+	probeCache[key] = probeCacheEntry{
+		probe:   probe,
+		expires: time.Now().Add(probeCacheTTL),
+	}
+	probeCacheMu.Unlock()
+}
+
+// ResetProbeCache clears the in-memory probe cache. Test-only.
+func ResetProbeCache() {
+	probeCacheMu.Lock()
+	probeCache = make(map[probeCacheKey]probeCacheEntry)
+	probeCacheMu.Unlock()
+}
+
+// ProbeCacheSize returns the number of entries currently cached. Exposed
+// for diagnostics + tests.
+func ProbeCacheSize() int {
+	probeCacheMu.RLock()
+	defer probeCacheMu.RUnlock()
+	return len(probeCache)
+}