fix(prod): payment/tax gateways never fake success outside Development

Production-readiness audit fixes — every mock fallback is now gated on
IsDevelopment; in production these paths fail loudly instead:

- ZarinPal/Tara/SnappPay init: missing credentials returned a MOCK
  payment URL whose callback verified as paid — a café could activate a
  paid plan without paying. Now: "Payment gateway is not configured."
- Tara/SnappPay verify: a forged MOCK-* trace/token on the callback was
  accepted as a verified payment in any environment. Now rejected
  outside Development.
- Taraz (سامانه مودیان): returned a fake MOCK-TARAZ tracking code as if
  invoices reached the tax authority. Now returns an honest error (the
  real integration is not built yet).
- Admin integrations: NextPay/Vandar removed — they were listed but have
  no gateway implementation (selecting them silently used ZarinPal).
- docker-compose: ASPNETCORE_ENVIRONMENT default flipped Development →
  Production so a missing env var can never run prod in dev mode.

86 tests pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docker-compose.admin.yml

@@ -26,7 +26,7 @@ services:
       redis:
         condition: service_healthy
     environment:
-      ASPNETCORE_ENVIRONMENT: "${ASPNETCORE_ENVIRONMENT:-Development}"
+      ASPNETCORE_ENVIRONMENT: "${ASPNETCORE_ENVIRONMENT:-Production}"
       ASPNETCORE_URLS: http://+:8080
       RUN_MIGRATIONS: "${RUN_MIGRATIONS:-true}"
       ConnectionStrings__DefaultConnection: "${DB_CONNECTION_STRING:-Host=postgres;Port=5432;Database=meezi;Username=meezi;Password=meezi_local_pass}"