Add idempotency module: Idempotency-Key for POST APIs on Spring Boot 4.1.2 with Redis and Postgres stores

One servlet filter, two interchangeable stores, 18 tests (9 scenarios x 2 stores) against a real
redis-server 7.0.15 and PostgreSQL 16.15, and 20 captured transcripts under output/: replay, 422 on a
changed body, concurrent duplicates, expiry, the crash window and a handler slower than the lock.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b5yg8grY17JsVAJviV

README.md

@@ -9,6 +9,7 @@ files.
 
 | Directory | Article | What it demonstrates |
 |---|---|---|
+| [`idempotency/`](idempotency) | [Idempotency Keys for POST APIs in Spring Boot (Stripe-Style) with Redis and Postgres](https://ankurm.com/idempotency-keys-spring-boot-redis-postgres/) | one servlet filter and two interchangeable stores: a retried POST after a client timeout creating two payments without a key and one with; 400 / 422 / 409 and replay with `Idempotent-Replayed`; ten simultaneous duplicates (nine `409` on Redis, nine replays on Postgres because the second `INSERT` waits on the first's uncommitted row); a crash after the payment commits leaving Redis with a stuck lock and then a duplicate payment, and Postgres with neither; a handler slower than the Redis lock finishing with nothing stored; the legacy `Asia/Calcutta` zone id that PostgreSQL 16 refuses at connect time |
 | [`http-service-clients/`](http-service-clients) | [Spring Boot 4 HTTP Service Clients: @HttpExchange Interfaces with RestClient (Complete Guide)](https://ankurm.com/spring-boot-4-http-service-clients/) | `@HttpExchange` interfaces registered with `@ImportHttpServices` and configured under `spring.http.serviceclient.<group>.*` (the keys the first version of the post gave are silently ignored), run against a local stub upstream: `defaultStatusHandler` with ProblemDetail decoding, `NotFoundRestClientAdapterDecorator` for `Optional` on a 404, Framework 7 `@Retryable` on the interface vs `RetryTemplate` vs an interceptor vs Resilience4j (call counts side by side, and a retried POST created twice), bearer, API-key and OAuth2 client-credentials auth (`@ClientRegistrationId` ignored without its group configurer), `MockRestServiceServer`, and the `@RestClientTest` slice where a group client is not bound to the mock |
 | [`pdf-invoice/`](pdf-invoice) | [Generate PDF Invoices in Spring Boot: OpenPDF vs iText vs Flying Saucer (HTML to PDF)](https://ankurm.com/spring-boot-generate-pdf-invoice-openpdf-itext-flying-saucer/) | one invoice rendered by OpenPDF 3.0.5, iText Core 9.8.0 (AGPL 3 or commercial) and Flying Saucer 10.7.0 from a Thymeleaf template, with embedded Noto Sans Devanagari: none of the three shapes Marathi/Hindi conjuncts by default (checked by rendering to PNG against a libraqm reference), OpenPDF's `GlyphLayoutManager` does but breaks text extraction until `ActualText` is on, OpenPDF's static `LayoutProcessor` is JVM-global and breaks other writers, Apache FOP on the classpath did not help, streamed vs buffered failure (`200` + curl exit 18 vs `500`), font subsetting, indicative per-invoice timings |
 | [`filter-interceptor-aop/`](filter-interceptor-aop) | [Filter vs HandlerInterceptor vs AOP in Spring Boot 4: Which Runs When](https://ankurm.com/spring-boot-filter-vs-handlerinterceptor-vs-aop-which-runs-when/) | the real order of a servlet filter, a `HandlerInterceptor` and an `@Aspect` printed from a running Tomcat and from MockMvc, the callbacks that still fire (and the ones that do not) after an exception, a 404, a `preHandle` that returns false and an async request, `afterCompletion` seeing `ex=null` after `@ControllerAdvice`, a header added in `postHandle` silently lost, a request body read twice, `ContentCachingResponseWrapper` returning an empty 200, Spring Security 7's chain sitting at order -100, a filter bean running twice, the Framework 7 path-matching removals read with `javap`, and the three self-invocation-style cases an aspect never sees |