internal/thumbs is internal/media, with Inspect: what a file holds, from ffprobe

The package was named for its first job; it now also decides what a file is for the
Files tab (image, video, audio) and reads its streams, which the viewer will need. Inspect
is the richer probe — codec, profile, pixel format, size, frame rate, HDR transfer, the
audio beside it, container and duration — run as root through sudo when asked.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/media/owner.go

@@ -0,0 +1,26 @@
+package media
+
+import (
+	"fmt"
+	"os"
+	"os/user"
+	"strconv"
+	"syscall"
+)
+
+// ownerOf is "root:render 0660" for a device node, the thing to say when it cannot be opened.
+func ownerOf(st os.FileInfo) string {
+	sys, ok := st.Sys().(*syscall.Stat_t)
+	if !ok {
+		return ""
+	}
+	u := strconv.Itoa(int(sys.Uid))
+	if x, err := user.LookupId(u); err == nil {
+		u = x.Username
+	}
+	g := strconv.Itoa(int(sys.Gid))
+	if x, err := user.LookupGroupId(g); err == nil {
+		g = x.Name
+	}
+	return fmt.Sprintf("%s:%s %04o", u, g, st.Mode().Perm())
+}