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.go

@@ -0,0 +1,77 @@
+// Package video prepares uploaded video for browser playback using the external
+// ffmpeg/ffprobe toolchain. Availability is detected at startup; when the tools
+// are absent the rest of the application keeps working and video upload is
+// disabled.
+package video
+
+import (
+	"os/exec"
+
+	"galo/internal/config"
+)
+
+// Toolchain locates the external video tools and reports whether they are
+// usable. A nil Toolchain is treated as unavailable.
+type Toolchain struct {
+	ffmpegPath  string
+	ffprobePath string
+	tempDir     string
+	available   bool
+}
+
+// Detect resolves the configured tool paths and records whether both binaries
+// are present. Missing tools are not an error: video is simply unavailable.
+func Detect(cfg config.Video) *Toolchain {
+	t := &Toolchain{tempDir: cfg.TempDir}
+	if p, err := exec.LookPath(cfg.FFmpegPath); err == nil {
+		t.ffmpegPath = p
+	} else {
+		t.ffmpegPath = cfg.FFmpegPath
+	}
+	if p, err := exec.LookPath(cfg.FFprobePath); err == nil {
+		t.ffprobePath = p
+	} else {
+		t.ffprobePath = cfg.FFprobePath
+	}
+	t.available = t.ffmpegPath != "" && t.ffprobePath != "" &&
+		isExecutable(t.ffmpegPath) && isExecutable(t.ffprobePath)
+	return t
+}
+
+func isExecutable(path string) bool {
+	if path == "" {
+		return false
+	}
+	if _, err := exec.LookPath(path); err == nil {
+		return true
+	}
+	return false
+}
+
+// Available reports whether both ffmpeg and ffprobe are usable.
+func (t *Toolchain) Available() bool { return t != nil && t.available }
+
+// FFmpegPath returns the resolved ffmpeg binary path.
+func (t *Toolchain) FFmpegPath() string {
+	if t == nil {
+		return ""
+	}
+	return t.ffmpegPath
+}
+
+// FFprobePath returns the resolved ffprobe binary path.
+func (t *Toolchain) FFprobePath() string {
+	if t == nil {
+		return ""
+	}
+	return t.ffprobePath
+}
+
+// TempDir returns the configured temporary directory (empty means the system
+// default).
+func (t *Toolchain) TempDir() string {
+	if t == nil {
+		return ""
+	}
+	return t.tempDir
+}