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/media/thumbs.go

@@ -62,6 +62,24 @@ type Caps struct {
 type Opts struct {
 	Size    int           // the long edge in pixels; 256 when zero
 	Timeout time.Duration // for the whole job, ffprobe and every ffmpeg attempt; 30 s when zero
+	Kind    string        // image | video | audio; from the name when empty (a URL has none)
+}
+
+// Only is the caps with one device left to decode on: a render node, CPU for none, or ""
+// for all of them — the Transcode device setting, which previews follow too. A device this
+// machine doesn't have leaves the CPU.
+func (c Caps) Only(device string) Caps {
+	if device == "" {
+		return c
+	}
+	out := c
+	out.GPUs = nil
+	for _, g := range c.GPUs {
+		if g.Device == device {
+			out.GPUs = append(out.GPUs, g)
+		}
+	}
+	return out
 }
 
 func (o Opts) defaults() Opts {
@@ -81,6 +99,8 @@ type Result struct {
 	Codec  string `json:"codec"`
 	Width  int    `json:"width"`
 	Height int    `json:"height"`
+	// Failure is why the file can't be previewed, when another machine tried (no JPEG then).
+	Failure string `json:"-"`
 }
 
 // ErrUnsupported: not an image or a video by its name.
@@ -356,7 +376,10 @@ func (a attempt) engine() string {
 
 // Make produces the preview of one file.
 func Make(ctx context.Context, c Caps, path string, o Opts) (Result, error) {
-	kind := Kind(path)
+	kind := o.Kind
+	if kind == "" {
+		kind = Kind(path)
+	}
 	if kind == "" {
 		return Result{}, ErrUnsupported
 	}