Add mongoUri and admin.{user,password} config keys to config/env/all.js
for Task 2 admin transaction management feature. Follows existing env var
pattern used for payment provider credentials. Default mongoUri points to
Docker service; admin credentials read from ADMIN_USER/ADMIN_PASSWORD env.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

11 bite-sized tasks: Docker Compose mongo service, config, Mongoose
connection, shared MegaPay signing helper, model, Basic Auth middleware,
create/pay/return/ipn/history endpoints, final Docker integration test.

New self-contained module for staff to create Epay payment links for
customers and track their status, kept separate from the just-stabilized
/epay/* customer-facing flow to avoid regression risk.