Files: re-encode a video the browser can't play, on the machine's own GPU, and play that

The viewer tries the file as it is first. When the browser gives up — Safari and an MKV,
HEVC without a hardware decoder — or on a tap, a picker: codec (H.264, HEVC, AV1, or the
original codec repackaged) and size (original, 1080p, 720p, 480p), each codec labelled
with the engine this machine would use. POST /api/files/play runs ffmpeg into an HLS
event playlist of fragmented-MP4 segments under <data dir>/transcode/<id>/; Safari plays
HLS itself and everything else goes through hls.js, loaded only then. The recipes are
tried best first: a card on the bus before the integrated GPU, decoding on the same card
before decoding on the CPU and uploading, the software encoders last; a recipe that
produces no first segment is dropped for the next. The seekable range grows as ffmpeg
gets ahead; status says how far. Streams are reused for the same request, closed when
the player goes, swept after ten idle minutes, and the folder is emptied at start. Root
mode runs ffmpeg as root through a helper that ends when the daemon lets go of it.

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

internal/media/thumbs_test.go

@@ -41,19 +41,31 @@ func TestParseVainfo(t *testing.T) {
       VAProfileH264High               :	VAEntrypointEncSliceLP
       VAProfileJPEGBaseline           :	VAEntrypointVLD
       VAProfileHEVCMain10             :	VAEntrypointVLD
+      VAProfileHEVCMain10             :	VAEntrypointEncSlice
       VAProfileHEVCMain444_12         :	VAEntrypointVLD
       VAProfileVP9Profile2            :	VAEntrypointVLD
       VAProfileAV1Profile0            :	VAEntrypointEncSliceLP
 `
-	got := parseVainfo(text)
-	want := []string{"h264", "hevc", "mjpeg", "mpeg2video", "vp9"}
-	if len(got) != len(want) {
-		t.Fatalf("got %v, want %v", got, want)
-	}
-	for i := range want {
-		if got[i] != want[i] {
-			t.Fatalf("got %v, want %v", got, want)
+	dec, enc := parseVainfo(text)
+	same := func(got, want []string) bool {
+		if len(got) != len(want) {
+			return false
 		}
+		for i := range want {
+			if got[i] != want[i] {
+				return false
+			}
+		}
+		return true
+	}
+	if !same(dec, []string{"h264", "hevc", "mjpeg", "mpeg2video", "vp9"}) {
+		t.Fatalf("decodes = %v", dec)
+	}
+	if !same(enc, []string{"av1", "h264", "hevc"}) {
+		t.Fatalf("encodes = %v", enc)
+	}
+	if d, e := parseVainfo(""); d != nil || e != nil {
+		t.Fatal("nothing from nothing")
 	}
 }