[og] Route crawlers via middleware, not a rewrite

The OG function deployed and worked (`/api/og?v=<nevent>` returns the real
title, description, thumbnail and player card), but crawlers hitting
`/?v=<nevent>` still got the site-wide card: Vercel evaluates `rewrites` AFTER
the filesystem, and `/` matches the static index.html, so the rewrite never
fired.

Middleware runs before the filesystem check, so that is where the routing
belongs. Replace the dead `rewrites` block with a root middleware that matches
only `/`, bails immediately when there is no `v` param or the user-agent is not
a known preview crawler, and otherwise rewrites to /api/og.

Real viewers are unaffected -- they fall through to the untouched SPA. Note the
one behavioral difference from the rewrite design: middleware now executes on
homepage requests (matcher is `/` only) rather than being invoked solely for
crawlers. It returns next() after two cheap checks.

Verified on the live unstable deployment that /api/og already renders
`x-bitvid-og: video` with og:type video.other for the real note; this only
fixes how crawlers reach it.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

package.json

@@ -85,6 +85,7 @@
   "dependencies": {
     "@floating-ui/dom": "^1.7.4",
     "@tailwindcss/typography": "^0.5.19",
+    "@vercel/edge": "^1.3.1",
     "autoprefixer": "^10.4.24",
     "cross-env": "^10.1.0",
     "esbuild": "^0.27.3",