OTP application callback for Sigra.
Sigra itself does not start any long-lived processes — it is a library whose security primitives run inline in the host application. This callback exists so that one-shot startup diagnostics (configuration checks, missing optional-dep warnings) can run when the host app boots.
Today it performs a single boot-time check: if the host has configured
config :sigra, :audit, retention_days: N but does not have Oban in the
dep tree, log a warning advising the inline fallback path
(Sigra.Audit.cleanup/1). See Phase 9 decisions D-09, D-10, D-36.