feat(video): add video upload with remux/transcode playback

Add video as a media kind in photos: streaming upload with per-kind
limits, ffprobe metadata, poster frame reusing the image variant
pipeline, and a playback object (remux when web-compatible, else
h264/yuv420p/aac transcode). Video jobs run on a separate pool with a
long lease. Frontend gains a video viewer, grid badge/duration and
video upload.

backend/internal/video/toolchain_test.go

@@ -0,0 +1,45 @@
+package video
+
+import (
+	"os"
+	"testing"
+
+	"galo/internal/config"
+)
+
+func TestDetectUnavailable(t *testing.T) {
+	tc := Detect(config.Video{
+		FFmpegPath:  "definitely-missing-ffmpeg-xyz",
+		FFprobePath: "definitely-missing-ffprobe-xyz",
+	})
+	if tc.Available() {
+		t.Fatalf("expected unavailable toolchain, got %+v", tc)
+	}
+}
+
+func TestDetectAvailable(t *testing.T) {
+	self, err := os.Executable()
+	if err != nil {
+		t.Fatalf("executable: %v", err)
+	}
+	tc := Detect(config.Video{FFmpegPath: self, FFprobePath: self, TempDir: "/tmp"})
+	if !tc.Available() {
+		t.Fatalf("expected available toolchain, got %+v", tc)
+	}
+	if tc.FFmpegPath() != self || tc.FFprobePath() != self {
+		t.Fatalf("paths = %q %q", tc.FFmpegPath(), tc.FFprobePath())
+	}
+	if tc.TempDir() != "/tmp" {
+		t.Fatalf("temp dir = %q", tc.TempDir())
+	}
+}
+
+func TestNilToolchain(t *testing.T) {
+	var tc *Toolchain
+	if tc.Available() {
+		t.Fatal("nil toolchain must be unavailable")
+	}
+	if tc.FFmpegPath() != "" || tc.FFprobePath() != "" || tc.TempDir() != "" {
+		t.Fatal("nil toolchain accessors must be empty")
+	}
+}