feat(download): add dynamic OG metadata and fix thumbnail caching

- Register a new route for box Open Graph images (`/d/{boxID}/og-image.jpg`).
- Dynamically set the download page title, description, and OG image URL based on box state (e.g., file count, expiration, password protection).
- Introduce `servePlaceholderThumbnail` to serve fallback thumbnails with `Cache-Control: no-store, must-revalidate`. This ensures the browser requests the real thumbnail once it is generated instead of caching the placeholder.

backend/libs/handlers/ogimage.go

@@ -0,0 +1,176 @@
+package handlers
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/draw"
+	_ "image/gif"
+	"image/jpeg"
+	_ "image/png"
+	"net/http"
+	"os"
+	"path/filepath"
+	"time"
+
+	xdraw "golang.org/x/image/draw"
+	_ "golang.org/x/image/webp"
+)
+
+// Open Graph image dimensions recommended for large summary cards
+// (Discord, Twitter/X, Slack, etc.).
+const (
+	ogImageWidth  = 1200
+	ogImageHeight = 630
+	ogMaxTiles    = 4
+	ogTileGap     = 8
+)
+
+var ogBackground = color.RGBA{R: 0x0b, G: 0x0b, B: 0x16, A: 0xff}
+
+// BoxOGImage renders the social-preview image for a box: a collage of up to
+// four file thumbnails, or a branded placeholder when none are available yet.
+func (a *App) BoxOGImage(w http.ResponseWriter, r *http.Request) {
+	box, err := a.uploadService.GetBox(r.PathValue("boxID"))
+	if err != nil {
+		http.NotFound(w, r)
+		return
+	}
+	if err := a.uploadService.CanDownload(box); err != nil {
+		a.serveOGImage(w, r, a.ogPlaceholder())
+		return
+	}
+
+	// Never leak thumbnails of a locked, obfuscated box. (Protected-but-not-
+	// obfuscated boxes already show their thumbnails on the download page, so
+	// they may appear here too.)
+	hideContents := a.uploadService.IsProtected(box) && box.Obfuscate
+
+	thumbs := make([]image.Image, 0, ogMaxTiles)
+	if !hideContents {
+		for _, file := range box.Files {
+			if len(thumbs) >= ogMaxTiles {
+				break
+			}
+			if file.Thumbnail == "" && file.ThumbnailObjectKey == "" {
+				continue
+			}
+			object, err := a.uploadService.OpenThumbnailObject(r.Context(), box, file)
+			if err != nil {
+				continue
+			}
+			img, _, decodeErr := image.Decode(object.Body)
+			object.Body.Close()
+			if decodeErr == nil {
+				thumbs = append(thumbs, img)
+			}
+		}
+	}
+
+	if len(thumbs) == 0 {
+		a.serveOGImage(w, r, a.ogPlaceholder())
+		return
+	}
+	a.serveOGImage(w, r, renderCollage(thumbs))
+}
+
+func (a *App) serveOGImage(w http.ResponseWriter, r *http.Request, img image.Image) {
+	var buf bytes.Buffer
+	if err := jpeg.Encode(&buf, img, &jpeg.Options{Quality: 85}); err != nil {
+		http.Error(w, "could not render preview image", http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "image/jpeg")
+	// Social scrapers fetch this rarely and cache on their side; a modest cache
+	// keeps it fresh as thumbnails finish generating.
+	w.Header().Set("Cache-Control", "public, max-age=3600")
+	http.ServeContent(w, r, "og-image.jpg", time.Time{}, bytes.NewReader(buf.Bytes()))
+}
+
+// ogPlaceholder builds the branded fallback image: the file placeholder icon
+// centered on the brand background.
+func (a *App) ogPlaceholder() image.Image {
+	canvas := image.NewRGBA(image.Rect(0, 0, ogImageWidth, ogImageHeight))
+	draw.Draw(canvas, canvas.Bounds(), &image.Uniform{ogBackground}, image.Point{}, draw.Src)
+
+	file, err := os.Open(filepath.Join(a.cfg.StaticDir, "img", "file-placeholder.webp"))
+	if err != nil {
+		return canvas
+	}
+	defer file.Close()
+	icon, _, err := image.Decode(file)
+	if err != nil {
+		return canvas
+	}
+
+	// Scale the icon to ~40% of the canvas height and centre it.
+	target := ogImageHeight * 2 / 5
+	b := icon.Bounds()
+	scale := float64(target) / float64(b.Dy())
+	dw := int(float64(b.Dx()) * scale)
+	dh := target
+	x0 := (ogImageWidth - dw) / 2
+	y0 := (ogImageHeight - dh) / 2
+	xdraw.CatmullRom.Scale(canvas, image.Rect(x0, y0, x0+dw, y0+dh), icon, b, xdraw.Over, nil)
+	return canvas
+}
+
+// renderCollage tiles up to four thumbnails into the OG canvas with a small gap.
+func renderCollage(thumbs []image.Image) image.Image {
+	canvas := image.NewRGBA(image.Rect(0, 0, ogImageWidth, ogImageHeight))
+	draw.Draw(canvas, canvas.Bounds(), &image.Uniform{ogBackground}, image.Point{}, draw.Src)
+
+	cols, rows := collageGrid(len(thumbs))
+	cellW := (ogImageWidth - ogTileGap*(cols+1)) / cols
+	cellH := (ogImageHeight - ogTileGap*(rows+1)) / rows
+
+	i := 0
+	for ry := 0; ry < rows && i < len(thumbs); ry++ {
+		for cx := 0; cx < cols && i < len(thumbs); cx++ {
+			x0 := ogTileGap + cx*(cellW+ogTileGap)
+			y0 := ogTileGap + ry*(cellH+ogTileGap)
+			drawCover(canvas, image.Rect(x0, y0, x0+cellW, y0+cellH), thumbs[i])
+			i++
+		}
+	}
+	return canvas
+}
+
+func collageGrid(n int) (cols, rows int) {
+	switch {
+	case n <= 1:
+		return 1, 1
+	case n == 2:
+		return 2, 1
+	case n == 3:
+		return 3, 1
+	default:
+		return 2, 2
+	}
+}
+
+// drawCover scales src to completely fill dst, cropping the overflow (centred),
+// preserving aspect ratio — the CSS object-fit: cover equivalent.
+func drawCover(dst *image.RGBA, cell image.Rectangle, src image.Image) {
+	b := src.Bounds()
+	iw, ih := b.Dx(), b.Dy()
+	if iw <= 0 || ih <= 0 {
+		return
+	}
+	cellAR := float64(cell.Dx()) / float64(cell.Dy())
+	imgAR := float64(iw) / float64(ih)
+
+	var sw, sh int
+	if imgAR > cellAR {
+		// Source is wider than the cell: crop the sides.
+		sh = ih
+		sw = int(float64(ih) * cellAR)
+	} else {
+		// Source is taller: crop top/bottom.
+		sw = iw
+		sh = int(float64(iw) / cellAR)
+	}
+	sx := b.Min.X + (iw-sw)/2
+	sy := b.Min.Y + (ih-sh)/2
+	xdraw.CatmullRom.Scale(dst, cell, src, image.Rect(sx, sy, sx+sw, sy+sh), xdraw.Over, nil)
+}