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/static/site.webmanifest

@@ -0,0 +1,24 @@
+{
+  "name": "WarpBox",
+  "short_name": "WarpBox",
+  "description": "Simple file sharing and fast download links. Upload files, generate share links, and serve clean download pages.",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#0b0b16",
+  "theme_color": "#8b5cf6",
+  "icons": [
+    {
+      "src": "/static/android-chrome-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/static/android-chrome-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}