[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-lock.json

@@ -11,6 +11,7 @@
       "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",
@@ -1478,6 +1479,12 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@vercel/edge": {
+      "version": "1.3.1",
+      "resolved": "https://registry.npmjs.org/@vercel/edge/-/edge-1.3.1.tgz",
+      "integrity": "sha512-k0tKWeYEWOv6qU6+Z8+hv2Nt/u3vFHRZdB0uvwE0DxoddU/5kWX/Chu8F6ojvqmRlnZwuX/7kyZFVeCP6DxqBg==",
+      "license": "Apache-2.0"
+    },
     "node_modules/@vitest/expect": {
       "version": "4.0.18",
       "resolved": "https://registry.npmjs.org/@vitest/expect/-/expect-4.0.18.tgz",