ci: inject throwaway auth secrets into smoke boot

Production validation rejects the compose placeholder secrets, so the
backend panicked at config load and the container went unhealthy.
Generate ephemeral BETTER_AUTH_SECRET and BETTER_AUTH_INTERNAL_TOKEN
per run; verified locally that the container reaches healthy.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.github/workflows/ci.yml

@@ -86,6 +86,8 @@ jobs:
 
       - name: Boot stack
         run: |
+          export BETTER_AUTH_SECRET=$(openssl rand -hex 32)
+          export BETTER_AUTH_INTERNAL_TOKEN=$(openssl rand -hex 32)
           docker compose -f infra/docker-compose.yml up -d --build postgres redis backend frontend traefik
 
       - name: Wait for backend health