test(ledger): prove the no-lost-update invariant under concurrent posting

[tiana-code]

What

Adds ConcurrentPostersIT, a Testcontainers PostgreSQL 17 concurrency test that proves the ledger's no-lost-update invariant under real contention. 100 virtual-thread posters, released together by a CountDownLatch start-gate, each commit a distinct balanced DEBIT +100.00 / CREDIT -100.00 transaction to one shared account pair through the idempotency-wrapped path (idempotencyService.execute { transactionService.post(...) }), the only path where the bounded optimistic-retry absorbs the @Version conflicts on the shared account_balances rows.

Asserted contract (non-flaky, S = observed success count)

• Final balance of each account equals the algebraic sum of the committed postings (balance == 100.00 * S via compareTo).
• Every poster resolves to exactly one outcome: committed success or a typed ConcurrencyConflictException; any other throwable (including a raw OptimisticLockingFailureException or a pool-timeout) is captured first and fails the test loudly.
• Transaction rows == S; entry rows == 2 * S; vacuous-run guard (balance != 0 when S >= 1).

All assertions are phrased over the observed S, so the test passes on every CI run regardless of how the scheduler resolves 100-way contention (default max-attempts=3, no backoff, means many posters legitimately resolve to a typed 503; that is correct and asserted).

Notes

• Test-only slice, zero production change.
• @DataJpaTest + @Transactional(NOT_SUPPORTED) so worker threads commit and the read-back sees committed cross-thread state; mirrors the AuditRetryTopologyIT harness.
• Hikari pool sized (maximum-pool-size=16, explicit connection-timeout) to keep real parallelism while staying within the shared-container budget; @AfterEach outboxRepository.deleteAll() keeps sibling global-count ITs green.
• Runs on CI only (Testcontainers; no in-memory fallback).

Closes #56