Add Backends For Frontends (BFF) pattern (#300)

[AnveshSrivastava]

Summary

Closes #300

Implements the Backends For Frontends (BFF) pattern as a new module,
backends-for-frontends. Two client-specific gateways — MobileBff and
DesktopBff — sit in front of a shared set of downstream microservices
(AuthService, CartService, OrderService, SupplierService) and each
aggregates only the services its own client needs, reshaping the result into
a response tailored to that client:

• MobileBff calls Auth + Cart + Order → returns a lean MobileDashboardResponse
  suited to a phone screen.
• DesktopBff calls Auth + Order + Supplier → returns a richer
  DesktopDashboardResponse with back-office detail a mobile client never needs.

This satisfies the issue's acceptance criteria: distinct client-specific backends,
data aggregation from multiple downstream services per BFF, and an optimized,
purpose-built API for each client.

What's included

• Domain models (Product, CartItem, User, Order, SupplierRecord)
• Downstream service interfaces + in-memory implementations standing in for
  real microservices
• ClientBff<T> contract + MobileBff / DesktopBff implementations
• App.java demonstrating both BFFs against the same user
• JUnit 5 tests for both BFFs and the demo entry point
• Class diagram (etc/backends-for-frontends.png + .urm.puml), referenced
  from the README
• README following the repo's standard pattern-page template

Verification

• mvn -pl backends-for-frontends -am clean test — all tests passing
• mvn checkstyle:check@validate -pl backends-for-frontends -am — 0 violations
• mvn spotless:check -pl backends-for-frontends -B — clean
• Coverage: 98% instructions, 100% methods, 100% classes (JaCoCo)
• Full reactor build (mvn clean install, module excluded from unrelated flakes
  below) — green

Notes for reviewers

• Two pre-existing, unrelated test failures were encountered while running the
  full reactor build and confirmed not caused by this change by reproducing
  them in isolation on a clean checkout:
  • rate-limiting-pattern's TokenBucketRateLimiterTest.shouldRefillTokensAfterTime
    is timing-flaky.
  • commander's test suite can hit OutOfMemoryError: unable to create native thread locally depending on OS thread limits, unrelated to this module.
• While verifying Checkstyle compliance, I found that WhitespaceAround's
  allowEmptyConstructors/allowEmptyTypes properties are unset (default
  false) in the current Checkstyle config, which conflicts with Spotless/
  Google Java Format's formatting of empty {} bodies. This affects any record
  with a fully empty body — I counted at least 16 other existing modules with
  the same latent construct, which only avoid failing because Checkstyle isn't
  bound to the install lifecycle. I worked around it locally in this module's
  6 records by giving each a minimal documented compact constructor rather than
  a bare empty body, but didn't touch the shared Checkstyle config or any other
  module — happy to open a separate follow-up issue for that if it's useful.

[github-actions]

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📥 Commits

Analyzing changes from base (2656d8e) to latest commit (3b1d5ca):

• 3b1d5ca: 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.

📁 Files being considered (2)

➕ backends-for-frontends/src/main/java/com/iluwatar/bff/service/SupplierService.java (1 hunk)
➕ backends-for-frontends/src/main/java/com/iluwatar/bff/service/impl/InMemorySupplierService.java (1 hunk)

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[codecov]

Codecov Report

❌ Patch coverage is 97.84946% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (74d2dbe) to head (3b1d5ca).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...iluwatar/bff/service/impl/InMemoryAuthService.java	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3543      +/-   ##
============================================
+ Coverage     83.24%   83.29%   +0.05%     
- Complexity     4025     4052      +27     
============================================
  Files          1060     1074      +14     
  Lines         14246    14339      +93     
  Branches        686      689       +3     
============================================
+ Hits          11859    11944      +85     
- Misses         2100     2107       +7     
- Partials        287      288       +1     

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