feat(backend): enhance social previews for single-file shares

Implements dynamic Open Graph (OG) metadata and image generation for
single-file shared boxes to improve social media previews.

Changes include:
- Added a new route `/d/{boxID}/f/{fileID}/og-image.jpg` for file-specific OG images.
- Updated `DownloadPage` to dynamically set the page title, description, and OG image properties when a box contains only one file.
- Restricted raw media inline serving for social bots to images and videos.
- Added helper functions to format file share descriptions and determine appropriate social image URLs and types.
- Integrated basic font rendering to support dynamic OG image generation.

backend/libs/jobs/thumbnails_test.go

@@ -4,12 +4,14 @@ import (
 	"bytes"
 	"image"
 	"image/color"
+	"image/jpeg"
 	"image/png"
 	"io"
 	"log/slog"
 	"mime/multipart"
 	"net/http/httptest"
 	"net/textproto"
+	"strings"
 	"testing"
 
 	"warpbox.dev/backend/libs/services"
@@ -46,6 +48,30 @@ func TestGenerateMissingThumbnailsForBox(t *testing.T) {
 	}
 }
 
+func TestCreateTextThumbnailRendersMarkdownAsJPEG(t *testing.T) {
+	data, err := createTextThumbnail(services.File{
+		Name:        "notes.md",
+		ContentType: "text/markdown",
+	}, strings.NewReader("# Meeting notes\n\n```go\nfunc main() {}\n```\n\nA rendered Markdown preview."))
+	if err != nil {
+		t.Fatalf("createTextThumbnail returned error: %v", err)
+	}
+
+	img, err := jpeg.Decode(bytes.NewReader(data))
+	if err != nil {
+		t.Fatalf("jpeg.Decode returned error: %v", err)
+	}
+	if img.Bounds().Dx() != 360 || img.Bounds().Dy() != 240 {
+		t.Fatalf("thumbnail size = %dx%d, want 360x240", img.Bounds().Dx(), img.Bounds().Dy())
+	}
+}
+
+func TestNeedsThumbnailIncludesCodeTextFiles(t *testing.T) {
+	if !needsThumbnail(services.File{Name: "main.go", ContentType: "text/plain"}) {
+		t.Fatalf("Go source file should get a text thumbnail")
+	}
+}
+
 func newThumbnailTestUploadService(t *testing.T) *services.UploadService {
 	t.Helper()
 	service, err := services.NewUploadService(1024*1024, t.TempDir(), "http://example.test", slog.New(slog.NewTextHandler(io.Discard, nil)))