Switch to Netlify adapter with GitHub API persistence

- Swap @astrojs/node for @astrojs/netlify adapter
- Admin API reads/writes admin-data.json via GitHub API in production
- Status changes (commitStatus) and deletes commit directly to repo
- Env vars: ADMIN_PASSWORD, GITHUB_TOKEN
- Local dev falls back to filesystem (no env vars needed)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -17,7 +17,7 @@
     "node": ">=22.12.0"
   },
   "dependencies": {
-    "@astrojs/node": "^10.0.4",
+    "@astrojs/netlify": "^7.0.7",
     "@astrojs/react": "^5.0.2",
     "@astrojs/rss": "^4.0.18",
     "@fontsource-variable/geist": "^5.2.8",
@@ -30,7 +30,7 @@
     "@types/katex": "^0.16.8",
     "@types/qrcode": "^1.5.6",
     "@types/three": "^0.183.1",
-    "astro": "^6.1.2",
+    "astro": "^6.1.7",
     "canvas-confetti": "^1.9.4",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",