feat: add HINDSIGHT_API_DEFAULT_BANK_TEMPLATE env var (#966)

* feat: add HINDSIGHT_API_DEFAULT_BANK_TEMPLATE env var

Server-level default bank template applied automatically to every
newly-created bank. Holds an inline JSON BankTemplateManifest with the
same shape as the /import endpoint body. Fields set by the template
become per-bank overrides so they take precedence over equivalent
HINDSIGHT_API_* env defaults. The template is applied once on first
creation and never reapplied, so user overrides via PATCH /config are
never clobbered. Malformed manifests are logged and ignored so a broken
server-level setting cannot wedge bank creation.

* chore: regenerate docs skill

* test: update async_retain test mock for renamed bank_profile helper