feat(seo): add robots.txt, sitemap, and noindex tags for downloads

Register routes for robots.txt and sitemap.xml, and implement search engine indexing controls to protect user privacy.

Specifically:
- Set `X-Robots-Tag: noindex, nofollow, noarchive` headers on file downloads, thumbnails, and zip generation.
- Configure `Robots: web.RobotsNone` on download and preview pages to prevent indexing of temporary user uploads.
- Add canonical URLs, improved descriptions, and image alt tags to page metadata for better social sharing.

backend/libs/handlers/app.go

@@ -134,6 +134,8 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}/download", a.DownloadFileContent)
 	mux.HandleFunc("GET /d/{boxID}/thumb/{fileID}", a.Thumbnail)
 	mux.HandleFunc("GET /d/{boxID}/og-image.jpg", a.BoxOGImage)
+	mux.HandleFunc("GET /robots.txt", a.RobotsTxt)
+	mux.HandleFunc("GET /sitemap.xml", a.SitemapXML)
 	mux.HandleFunc("GET /health", a.Health)
 	mux.HandleFunc("GET /healthz", notFound)
 	mux.HandleFunc("GET /api/v1/health", notFound)