feat: add automatic GPU detection for hardware encoding

Implemented automatic hardware encoder selection when hardware
acceleration is set to 'auto'. The system now detects the GPU
vendor and automatically selects the appropriate encoder:

- NVIDIA GPU → nvenc (h264_nvenc, hevc_nvenc, av1_nvenc)
- AMD GPU → amf (h264_amf, hevc_amf, av1_amf)
- Intel GPU → qsv (h264_qsv, hevc_qsv, av1_qsv)
- No compatible GPU → none (software encoding)

Detection uses the existing sysinfo.Detect() function which
identifies GPUs via nvidia-smi, lspci, wmic, and system_profiler
depending on the platform.

Location: main.go line 5867-5883

main.go

@@ -5862,6 +5862,26 @@ func buildFFmpegCommandFromJob(job *queue.Job) string {
 		// Determine codec (simplified)
 		codec := "libx264"
 		hardwareAccel, _ := cfg["hardwareAccel"].(string)
+
+		// Resolve "auto" to actual GPU vendor
+		if hardwareAccel == "auto" {
+			hwInfo := sysinfo.Detect()
+			switch hwInfo.GPUVendor {
+			case "nvidia":
+				hardwareAccel = "nvenc"
+				logging.Debug(logging.CatFFMPEG, "auto hardware accel resolved to nvenc (detected NVIDIA GPU)")
+			case "amd":
+				hardwareAccel = "amf"
+				logging.Debug(logging.CatFFMPEG, "auto hardware accel resolved to amf (detected AMD GPU)")
+			case "intel":
+				hardwareAccel = "qsv"
+				logging.Debug(logging.CatFFMPEG, "auto hardware accel resolved to qsv (detected Intel GPU)")
+			default:
+				hardwareAccel = "none"
+				logging.Debug(logging.CatFFMPEG, "auto hardware accel resolved to none (no compatible GPU detected)")
+			}
+		}
+
 		switch {
 		case videoCodec == "H.265" && hardwareAccel == "nvenc":
 			codec = "hevc_nvenc"