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/ffmpeg_test.go

@@ -0,0 +1,73 @@
+package video
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func argsContain(args []string, want ...string) bool {
+	for _, w := range want {
+		found := false
+		for _, a := range args {
+			if a == w {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return false
+		}
+	}
+	return true
+}
+
+func TestRemuxArgs(t *testing.T) {
+	args := RemuxArgs("in.mkv", "out.mp4")
+	if !argsContain(args, "-c", "copy", "-movflags", "+faststart", "-map", "0:a:0?") {
+		t.Fatalf("remux args = %v", args)
+	}
+}
+
+func TestTranscodeArgs(t *testing.T) {
+	args := TranscodeArgs("in.mov", "out.mp4", MaxLongSide)
+	if !argsContain(args, "libx264", "yuv420p", "aac", "-movflags", "+faststart") {
+		t.Fatalf("transcode args = %v", args)
+	}
+	var scale string
+	for i, a := range args {
+		if a == "-vf" && i+1 < len(args) {
+			scale = args[i+1]
+		}
+	}
+	if !strings.Contains(scale, "min(1920,iw)") || !strings.Contains(scale, "force_divisible_by=2") {
+		t.Fatalf("scale filter = %q", scale)
+	}
+}
+
+func TestPosterArgs(t *testing.T) {
+	args := PosterArgs("in.mp4", "poster.jpg", 1)
+	if !argsContain(args, "-ss", "1", "-frames:v", "1", "-f", "image2") {
+		t.Fatalf("poster args = %v", args)
+	}
+}
+
+func TestRun(t *testing.T) {
+	dir := t.TempDir()
+	ok := filepath.Join(dir, "ok")
+	if err := os.WriteFile(ok, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
+		t.Fatalf("write ok: %v", err)
+	}
+	if err := Run(context.Background(), ok, "-i", "x"); err != nil {
+		t.Fatalf("run ok: %v", err)
+	}
+	bad := filepath.Join(dir, "bad")
+	if err := os.WriteFile(bad, []byte("#!/bin/sh\necho nope >&2\nexit 3\n"), 0o755); err != nil {
+		t.Fatalf("write bad: %v", err)
+	}
+	if err := Run(context.Background(), bad); err == nil || !strings.Contains(err.Error(), "nope") {
+		t.Fatalf("run bad err = %v, want stderr captured", err)
+	}
+}