files: video previews are made on the Transcode device too

The setting's device makes every video preview: on the file's own machine
when it is there (the hub tells it by header), else on the device's machine,
which reads the file through the hub by a one-file token, and the preview is
kept in the file's machine's own cache (PUT /api/files/thumb, refused if the
file changed). Images and covers stay on the file's machine's CPU. If the
device's machine can't, the file's makes it as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -276,6 +276,9 @@ func (s *Server) routes() *http.ServeMux {
 	m.HandleFunc("POST /api/files/delete", s.auth(s.filesDelete))
 	m.HandleFunc("POST /api/files/chmod", s.auth(s.filesChmod))
 	m.HandleFunc("GET /api/files/thumb", s.auth(s.filesThumb))
+	m.HandleFunc("PUT /api/files/thumb", s.auth(s.thumbPut))
+	m.HandleFunc("POST /api/files/thumb-from", s.auth(s.thumbFrom))
+	m.HandleFunc("GET /api/nodes/{name}/files/thumb", s.auth(s.hubThumb))
 	m.HandleFunc("GET /api/files/thumbs", s.auth(s.thumbsStatus))
 	m.HandleFunc("POST /api/files/thumbs/install", s.auth(s.thumbsInstall))
 	m.HandleFunc("POST /api/files/thumbs/gpu-access", s.auth(s.thumbsGPUAccess))