migrate auth from next-auth to better-auth with magic link support

Replace next-auth with better-auth, adding magic link email login as
the primary auth method and GitHub OAuth as an optional social provider.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -14,19 +14,19 @@
     "@date-fns/tz": "^1.2.0",
     "@mdx-js/loader": "^3.1.1",
     "@mdx-js/react": "^3.1.1",
-    "@next-auth/mongodb-adapter": "^1.1.3",
     "@next/mdx": "^16.1.6",
     "@types/mdx": "^2.0.11",
     "@types/nodemailer": "^7.0.9",
     "@vercel/analytics": "^1.6.1",
     "autoprefixer": "^10.4.24",
+    "better-auth": "^1.4.18",
     "classnames": "^2.5.1",
     "date-fns": "^4.1.0",
     "googleapis": "^171.4.0",
     "ics": "^3.8.1",
+    "mongodb": "^7.1.0",
     "mongoose": "^9.2.1",
     "next": "^16.1.6",
-    "next-auth": "^4.24.13",
     "next-axiom": "^1.10.0",
     "nodemailer": "^8.0.1",
     "react": "^19.2.4",
@@ -53,4 +53,4 @@
       "^.+\\.(ts|tsx)$": "ts-jest"
     }
   }
-}
+}