The @session_options module attribute used Application.compile_env, which
baked compile-time placeholders into the endpoint while runtime.exs set
the real values from SESSION_SIGNING_SALT / SESSION_ENCRYPTION_SALT env
vars. Phoenix detected the mismatch and refused to start (failing migrate
Job in k8s).
- Remove hardcoded salts from config/config.exs (no compile-time binding)
- Add stable per-env salts in dev.exs / test.exs so local + CI don't need
the env vars
- Split static cookie opts (@static_session_options) from runtime-resolved
opts in endpoint.ex; expose session_options/0 as an MFA tuple in socket
connect_info so LiveView decodes sessions with the same runtime salts
- New ToweropsWeb.Plugs.RuntimeSession wraps Plug.Session, fetches salts
from app env on first request, and caches the initialized opts in
:persistent_term (zero per-request overhead after warm-up)
