fix: declare env vars in turbo.json so Vercel build receives them

Turbo was stripping env vars during build on Vercel. Added all required
vars to turbo.json build task. Also hardened Supabase and BetterAuth
init with fallback placeholders for build-time safety.

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

turbo.json

@@ -5,7 +5,18 @@
     "build": {
       "dependsOn": ["^build"],
       "inputs": ["$TURBO_DEFAULT$", ".env*"],
-      "outputs": [".next/**", "!.next/cache/**", "dist/**"]
+      "outputs": [".next/**", "!.next/cache/**", "dist/**"],
+      "env": [
+        "BETTER_AUTH_SECRET",
+        "BETTER_AUTH_URL",
+        "GOOGLE_CLIENT_ID",
+        "GOOGLE_CLIENT_SECRET",
+        "RESEND_API_KEY",
+        "POSTGRES_URL",
+        "NEXT_PUBLIC_SUPABASE_URL",
+        "NEXT_PUBLIC_SUPABASE_ANON_KEY",
+        "SUPABASE_SERVICE_ROLE_KEY"
+      ]
     },
     "lint": {
       "dependsOn": ["^lint"]