Add Jakarta Bean Validation to request DTOs

[tiana-code]

Context

Request DTOs must carry Jakarta Bean Validation annotations so that Spring's @Valid mechanism rejects structurally invalid input before it reaches the service layer. Common annotations: @NotNull, @NotBlank, @Size, @Pattern for strings; @Positive for amounts; @NotEmpty for entry lists. Validation errors are mapped to RFC 7807 400 responses by GlobalExceptionHandler.

Acceptance criteria

• Every non-optional field in every request DTO has an appropriate constraint annotation.
• currency fields are validated against ISO 4217 pattern (3 uppercase letters).
• entries list has @NotEmpty and @Size(min=2) to enforce the two-entry minimum before hitting the service.
• amount fields are validated with @Positive or @DecimalMin(value=0, inclusive=false).
• @WebMvcTest slice test asserts a missing required field returns 400 with a populated errors[] array.

References

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

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). Request DTOs CreateAccountRequest and PostTransactionRequest carry jakarta.validation constraints, both write controllers use @Valid @RequestBody, and GlobalExceptionHandler.handleValidation maps MethodArgumentNotValidException to a 400 RFC 7807 ProblemDetail with an errors array. Closing as done.