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/play_test.go

@@ -1,7 +1,9 @@
 package api
 
 import (
+	"fmt"
 	"net/http/httptest"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strings"
@@ -49,3 +51,43 @@ func TestPlaySrc(t *testing.T) {
 		t.Fatalf("unknown token: %d", w.Code)
 	}
 }
+
+// A preview made on another machine is kept by the file's own: only while the file is still
+// the one it was made of, and a failure is kept as a failure.
+func TestThumbPut(t *testing.T) {
+	dir := t.TempDir()
+	p := filepath.Join(dir, "film.mp4")
+	if err := os.WriteFile(p, []byte("0123456789"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	fi, _ := os.Stat(p)
+	s := &Server{fsLocal: files.Local{}, thumbs: newThumbSvc(dir, "")}
+	v := fmt.Sprintf("%d-%d", fi.ModTime().UnixMilli(), fi.Size())
+	put := func(v string, body string, hdr map[string]string) int {
+		r := httptest.NewRequest("PUT", "/api/files/thumb?path="+url.QueryEscape(p)+"&v="+v, strings.NewReader(body))
+		for k, x := range hdr {
+			r.Header.Set(k, x)
+		}
+		w := httptest.NewRecorder()
+		s.thumbPut(w, r)
+		return w.Code
+	}
+	get := func() *httptest.ResponseRecorder {
+		r := httptest.NewRequest("GET", "/api/files/thumb?cached=1&path="+url.QueryEscape(p)+"&v="+v, nil)
+		w := httptest.NewRecorder()
+		s.filesThumb(w, r)
+		return w
+	}
+	if w := get(); w.Code != 404 {
+		t.Fatalf("cached=1 before anything: %d", w.Code)
+	}
+	if c := put("1-10", "jpeg", nil); c != 409 {
+		t.Fatalf("a changed file: %d", c)
+	}
+	if c := put(v, "jpeg-bytes", map[string]string{"X-Hamlab-Thumb": "grid vaapi:/dev/dri/renderD129"}); c != 204 {
+		t.Fatalf("put: %d", c)
+	}
+	if w := get(); w.Code != 200 || w.Body.String() != "jpeg-bytes" || !strings.Contains(w.Header().Get("X-Hamlab-Thumb"), "grid") {
+		t.Fatalf("kept: %d %q %q", w.Code, w.Body, w.Header().Get("X-Hamlab-Thumb"))
+	}
+}