Add GlobalExceptionHandler returning RFC 7807 problem details

[tiana-code]

Context

GlobalExceptionHandler is a @RestControllerAdvice that catches all typed domain exceptions, Spring validation exceptions, and unexpected exceptions, and maps them to RFC 7807 Problem Details responses (Content-Type: application/problem+json). Every response includes a correlationId from the MDC context.

Acceptance criteria

• MethodArgumentNotValidException maps to 400 with an errors[] array containing field and code per failing constraint.
• Typed domain exceptions (NotFoundException, ConflictException, InvariantException) map to 404, 409, 422 respectively.
• OptimisticLockException after retries exhausted maps to 503.
• Unhandled exceptions map to 500 without stack trace in the response body (stack trace goes to structured logs only).
• Every response body includes correlationId matching the X-Correlation-Id response header.

References

Epic E-01 Ledger Foundation. Wiki: Code-Rules, API-Ledger, Architecture-Security.

Definition of Done

Tests green; ktlint and detekt clean; coverage target met; reviewed; SPDX header on new files; no maritime or real-provider names.

[tiana-code]

Already implemented in the F-01.5 REST slice (merged, PR #106). com.fincore.ledger.exception.GlobalExceptionHandler is a @RestControllerAdvice returning RFC 7807 ProblemDetail for every domain exception (AccountNotFound, TransactionNotFound, Duplicate/AlreadyReversed/IdempotencyConflict -> 409, Currency/DoubleEntry/Domain -> 422, ConcurrencyConflict -> 503 + Retry-After), plus MethodArgumentNotValid, HttpMessageNotReadable, IllegalArgument -> 400 and a 500 fallback. Refinement of the type URIs into a stable catalog + machine-readable error codes is tracked separately in #46. Closing as done.