Add GPU serial number extraction for NVIDIA diagnostics

Parse inventory/output.log to extract GPU serial numbers from lspci output,
expose them via serials API, and add GPU category to web UI.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/server/handlers.go

@@ -391,6 +391,24 @@ func (s *Server) handleGetSerials(w http.ResponseWriter, r *http.Request) {
 			})
 		}
 
+		// GPUs
+		for _, gpu := range result.Hardware.GPUs {
+			if gpu.SerialNumber == "" {
+				continue
+			}
+			model := gpu.Model
+			if model == "" {
+				model = "GPU"
+			}
+			serials = append(serials, SerialEntry{
+				Component:    model,
+				Location:     gpu.Slot,
+				SerialNumber: gpu.SerialNumber,
+				Manufacturer: gpu.Manufacturer,
+				Category:     "GPU",
+			})
+		}
+
 		// PCIe devices
 		for _, pcie := range result.Hardware.PCIeDevices {
 			if pcie.SerialNumber == "" {