feat: Add Backends For Frontends (BFF) pattern (#300) (#3543)

* Add Backends For Frontends pattern (#300)

Implements the BFF pattern with two client-specific gateways
(MobileBff, DesktopBff) aggregating shared downstream services
(AuthService, CartService, OrderService, SupplierService) into
client-tailored response shapes.

Closes #300

* Rename SupplierService lookup key to productName for clarity

The service is looked up by product name throughout the codebase, not
an actual product ID as the previous naming implied.

* Address review feedback: remove unnecessary compact constructors, add InMemory* service tests

pom.xml

@@ -80,6 +80,7 @@
     <module>anti-corruption-layer</module>
     <module>arrange-act-assert</module>
     <module>async-method-invocation</module>
+    <module>backends-for-frontends</module>
     <module>balking</module>
     <module>bloc</module>
     <module>bridge</module>