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

idempotency/src/main/resources/schema.sql

@@ -0,0 +1,20 @@
+CREATE TABLE IF NOT EXISTS payments (
+    id         BIGSERIAL PRIMARY KEY,
+    customer   TEXT        NOT NULL,
+    amount     BIGINT      NOT NULL,
+    currency   CHAR(3)     NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+-- Used only when idempotency.store=postgres.
+CREATE TABLE IF NOT EXISTS idempotency_keys (
+    idem_key        TEXT PRIMARY KEY,
+    fingerprint     CHAR(64)    NOT NULL,
+    status          TEXT        NOT NULL,
+    response_status INT,
+    content_type    TEXT,
+    response_body   TEXT,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
+    expires_at      TIMESTAMPTZ NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idempotency_keys_expires_at ON idempotency_keys (expires_at);