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/meta.go

@@ -0,0 +1,58 @@
+package handlers
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// RobotsTxt serves /robots.txt dynamically so the Sitemap URL reflects the
+// configured base URL rather than a hard-coded placeholder.
+func (a *App) RobotsTxt(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	w.Header().Set("Cache-Control", "public, max-age=86400")
+	fmt.Fprintf(w, `User-agent: *
+Allow: /
+
+# Private routes — do not crawl
+Disallow: /admin/
+Disallow: /api/
+Disallow: /app/
+Disallow: /account/
+Disallow: /d/*/f/*/download
+Disallow: /d/*/zip
+Disallow: /d/*/thumb/
+Disallow: /d/*/og-image.jpg
+Disallow: /d/*/unlock
+Disallow: /d/*/manage/
+
+Sitemap: %s/sitemap.xml
+`, strings.TrimRight(siteBaseURL(r, a.cfg.BaseURL), "/"))
+}
+
+// SitemapXML serves a minimal /sitemap.xml containing only the public,
+// indexable homepage. Box/file pages are noindex and deliberately excluded.
+func (a *App) SitemapXML(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+	w.Header().Set("Cache-Control", "public, max-age=3600")
+	baseURL := strings.TrimRight(siteBaseURL(r, a.cfg.BaseURL), "/")
+	lastMod := time.Now().UTC().Format("2006-01-02")
+	fmt.Fprintf(w, `<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>%s/</loc>
+    <lastmod>%s</lastmod>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+  </url>
+</urlset>
+`, baseURL, lastMod)
+}
+
+func siteBaseURL(r *http.Request, configured string) string {
+	if configured != "" {
+		return configured
+	}
+	return absoluteURL(r, "/")
+}