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/hwaccel_test.go

@@ -1,6 +1,9 @@
 package engine
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestHWAccelFFmpegVideoCodec(t *testing.T) {
 	cases := []struct {
@@ -32,3 +35,107 @@ func TestDetectHWAccelEmptyPathReturnsNone(t *testing.T) {
 		t.Errorf("got %s, want %s", got, HWAccelNone)
 	}
 }
+
+func TestResolveEncoderProfileDefaults(t *testing.T) {
+	cases := []struct {
+		hw         HWAccel
+		configured string
+		wantCodec  string
+		wantPreset string
+	}{
+		// Empty configured preset → pick latency-biased default per backend.
+		{HWAccelNone, "", "libx264", "superfast"},
+		{HWAccelNVENC, "", "h264_nvenc", "p3"},
+		{HWAccelQSV, "", "h264_qsv", "veryfast"},
+		// VideoToolbox has no preset knob — Preset should be "" regardless of input.
+		{HWAccelVideoToolbox, "p4", "h264_videotoolbox", ""},
+		{HWAccelVideoToolbox, "", "h264_videotoolbox", ""},
+		// VAAPI codec name resolved correctly; no preset substitution (uses "").
+		{HWAccelVAAPI, "", "h264_vaapi", ""},
+	}
+	for _, tc := range cases {
+		got := ResolveEncoderProfile(tc.hw, tc.configured)
+		if got.Codec != tc.wantCodec || got.Preset != tc.wantPreset {
+			t.Errorf("ResolveEncoderProfile(%s, %q) = {%s, %s}, want {%s, %s}",
+				tc.hw, tc.configured, got.Codec, got.Preset, tc.wantCodec, tc.wantPreset)
+		}
+	}
+}
+
+func TestResolveEncoderProfileHonoursConfiguredPreset(t *testing.T) {
+	// libx264 / NVENC / QSV all defer to the configured preset when set.
+	cases := []struct {
+		hw         HWAccel
+		configured string
+		wantPreset string
+	}{
+		{HWAccelNone, "ultrafast", "ultrafast"},
+		{HWAccelNVENC, "p1", "p1"},
+		{HWAccelQSV, "veryslow", "veryslow"},
+	}
+	for _, tc := range cases {
+		got := ResolveEncoderProfile(tc.hw, tc.configured)
+		if got.Preset != tc.wantPreset {
+			t.Errorf("ResolveEncoderProfile(%s, %q).Preset = %q, want %q",
+				tc.hw, tc.configured, got.Preset, tc.wantPreset)
+		}
+	}
+}
+
+func TestHWAccelDiagnosticLogLineNone(t *testing.T) {
+	d := HWAccelDiagnostic{
+		Pick:          HWAccelNone,
+		FFmpegPath:    "/usr/local/bin/ffmpeg",
+		FFmpegVersion: "ffmpeg version 6.1.1",
+		Encoders:      nil,
+		Devices:       nil,
+	}
+	line := d.LogLine()
+	wantSubstrings := []string{
+		"ffmpeg version 6.1.1",
+		"/usr/local/bin/ffmpeg",
+		"HW=none",
+		"software libx264",
+		"no HW encoders compiled in",
+	}
+	for _, want := range wantSubstrings {
+		if !strings.Contains(line, want) {
+			t.Errorf("expected substring %q in log line; got %q", want, line)
+		}
+	}
+}
+
+func TestHWAccelDiagnosticLogLineNVENCWithDevices(t *testing.T) {
+	d := HWAccelDiagnostic{
+		Pick:          HWAccelNVENC,
+		FFmpegPath:    "/usr/bin/ffmpeg",
+		FFmpegVersion: "ffmpeg version 6.0",
+		Encoders:      []string{"h264_nvenc", "hevc_nvenc", "h264_qsv"},
+		Devices:       []string{"/dev/nvidia0", "nvidia-smi"},
+	}
+	line := d.LogLine()
+	for _, want := range []string{"HW=nvenc", "h264_nvenc", "/dev/nvidia0", "nvidia-smi"} {
+		if !strings.Contains(line, want) {
+			t.Errorf("expected substring %q in log line; got %q", want, line)
+		}
+	}
+}
+
+func TestHWAccelDiagnosticLogLineSoftwareButEncodersFound(t *testing.T) {
+	// Edge case: ffmpeg compiled WITH nvenc but no /dev/nvidia0 (container w/o GPU).
+	// LogLine should flag the encoders so the user knows where the gap is.
+	d := HWAccelDiagnostic{
+		Pick:          HWAccelNone,
+		FFmpegPath:    "/usr/bin/ffmpeg",
+		FFmpegVersion: "ffmpeg version 6.0",
+		Encoders:      []string{"h264_nvenc"},
+		Devices:       nil,
+	}
+	line := d.LogLine()
+	for _, want := range []string{"HW=none", "encoders found but no matching device", "h264_nvenc"} {
+		if !strings.Contains(line, want) {
+			t.Errorf("expected substring %q in log line; got %q", want, line)
+		}
+	}
+}
+