feat(transcode): dynamic H.264 level + HW probe + capability reporting

Three related fixes around 4K-source transcoding that left the web
player stuck on "preparing session" with no useful diagnostics:

1. Dynamic -level:v derived from output height (hls.go, transcoder.go).
   The previous fixed "4.0" silently rejected anything taller than 1080p
   inside libx264 — "frame MB size > level limit", "DPB size > level
   limit" — and emitted unplayable segments. Helper H264LevelForHeight()
   now picks 4.0 / 5.0 / 5.1 / 6.0 from the actual encode height.

2. New `unarr probe-hwaccel` diagnostic command. Lists the HW encoders
   compiled into ffmpeg, the device files / drivers present, and the
   backend the daemon would actually pick today. Surfaces the canonical
   gotcha: a host with an RTX 3090 + nvidia-smi but a Homebrew ffmpeg
   built without --enable-nvenc still falls back to libx264 software.

3. Register payload now includes hwAccel + maxTranscodeHeight so the web
   side can suggest a smaller alternate quality before the user even
   tries to play a 4K source on a software-only host. Software-only =
   1080p cap, any HW backend = 2160p cap.

internal/agent/daemon.go

@@ -14,15 +14,17 @@ import (
 
 // DaemonConfig holds daemon runtime settings.
 type DaemonConfig struct {
-	AgentID     string
-	AgentName   string
-	Version     string
-	DownloadDir string
-	StreamPort  int      // port for the HTTP stream server
-	LanIP       string   // LAN IP (reported in sync for stream URL resolution)
-	TailscaleIP string   // Tailscale IP (reported in sync for stream URL resolution)
-	CanDelete   bool     // library.allow_delete is enabled
-	ScanPaths   []string // configured scan paths for file deletion validation
+	AgentID            string
+	AgentName          string
+	Version            string
+	DownloadDir        string
+	StreamPort         int      // port for the HTTP stream server
+	LanIP              string   // LAN IP (reported in sync for stream URL resolution)
+	TailscaleIP        string   // Tailscale IP (reported in sync for stream URL resolution)
+	CanDelete          bool     // library.allow_delete is enabled
+	ScanPaths          []string // configured scan paths for file deletion validation
+	HWAccel            string   // detected encoder backend ("nvenc"/"qsv"/"vaapi"/"videotoolbox"/"none")
+	MaxTranscodeHeight int      // resolution cap the agent can transcode comfortably (px)
 }
 
 // Daemon manages agent registration and the sync loop.
@@ -78,15 +80,17 @@ func (d *Daemon) UpdateStreamPort(port int) {
 // Retries with exponential backoff on transient errors (429, 5xx, network).
 func (d *Daemon) Register(ctx context.Context) error {
 	req := RegisterRequest{
-		AgentID:     d.cfg.AgentID,
-		Name:        d.cfg.AgentName,
-		OS:          runtime.GOOS,
-		Arch:        runtime.GOARCH,
-		Version:     d.cfg.Version,
-		DownloadDir: d.cfg.DownloadDir,
-		StreamPort:  d.cfg.StreamPort,
-		LanIP:       d.cfg.LanIP,
-		TailscaleIP: d.cfg.TailscaleIP,
+		AgentID:            d.cfg.AgentID,
+		Name:               d.cfg.AgentName,
+		OS:                 runtime.GOOS,
+		Arch:               runtime.GOARCH,
+		Version:            d.cfg.Version,
+		DownloadDir:        d.cfg.DownloadDir,
+		StreamPort:         d.cfg.StreamPort,
+		LanIP:              d.cfg.LanIP,
+		TailscaleIP:        d.cfg.TailscaleIP,
+		HWAccel:            d.cfg.HWAccel,
+		MaxTranscodeHeight: d.cfg.MaxTranscodeHeight,
 	}
 	if free, total, err := DiskInfo(d.cfg.DownloadDir); err == nil {
 		req.DiskFreeBytes = free

internal/agent/types.go

@@ -18,6 +18,14 @@ type RegisterRequest struct {
 	StreamPort     int    `json:"streamPort,omitempty"`
 	LanIP          string `json:"lanIp,omitempty"`
 	TailscaleIP    string `json:"tailscaleIp,omitempty"`
+	// Transcode capabilities — let the web side suggest a smarter quality
+	// before the player even starts. HWAccel is the picked backend
+	// ("nvenc"/"qsv"/"vaapi"/"videotoolbox"/"none"). MaxTranscodeHeight is
+	// the largest output resolution the agent can encode comfortably; for
+	// software-only ffmpeg this is 1080p, with a real GPU encoder it goes
+	// up to 2160p.
+	HWAccel            string `json:"hwAccel,omitempty"`
+	MaxTranscodeHeight int    `json:"maxTranscodeHeight,omitempty"`
 }
 
 // RegisterResponse is returned by the server after registration.