chore(ci): update placeholders in CI configuration for Next.js build

- Modified environment variable comments to clarify their purpose for prerendering in Next.js.
- Added a new placeholder for BETTER_AUTH_SECRET to ensure proper configuration during CI builds.

Author: kzndotsh

.github/workflows/ci.yml

@@ -114,8 +114,9 @@ jobs:
     permissions:
       contents: read
     env:
-      # Placeholder so db client loads during Next.js build; routes that use DB are force-dynamic
+      # Placeholders so Next.js build can prerender; not used for real requests
       DATABASE_URL: postgresql://localhost:5432/__build_placeholder__
+      BETTER_AUTH_SECRET: __ci_build_placeholder_do_not_use_in_production__
     steps:
       - name: Checkout
         uses: actions/checkout@v6