fix(config): declare kura backend in release configs

[Taure]

Problem

Self-host users following https://asobi.dev/docs/self-host pull ghcr.io/widgrensit/asobi_lua:latest and the container crashes on startup:

{application_start_failure,kura,
  {bad_return,{{kura_app,start,[normal,[]]},
    {'EXIT',{{no_pool_module_configured,asobi_repo}, ...}}}}}

Postgres stays up and idle because asobi never gets far enough to dial it.

Root cause

kura v2 (adopted in the kura v2 bump) resolves a repo's pool/driver module from its {backend, _} key via kura_app:resolve_backends/0. The release configs baked into the published image lost that key, so kura never sets pool_module and kura_db:get_pool_module/1 errors.

CI is green because it boots the dev config; the bug only existed in the configs baked into the release image.

Fix

• Add {backend, kura_backend_postgres} to config/prod_sys.config.src and config/dev_sys.config.src (resolves to pool_module => kura_pool_pgo, dialect => kura_dialect_pg).
• Add asobi_lua_release_config_tests — parses the release configs and asserts the kura repo declares a resolvable backend, closing the gap where prod-config drift shipped without any test exercising it.

After merge the asobi_lua:latest image must be republished so docker compose pull picks up the fix.