Address PR review feedback: fix repository interface, documentation, and validation

[Copilot]

Description

Addresses all review comments from PR #404. The build was failing due to missing CRUD operations in StatementRepository, and several documentation and code quality issues were identified.

Resolves #404 (review feedback)

Changes

Repository Interface

• Extended StatementRepository with both PagingAndSortingRepository and CrudRepository to restore findById(), save(), and saveAll() methods

StatementEntity

• Made stored column non-nullable to prevent ordering inconsistencies
• Added JavaDoc for constructor parameter and getter/setter methods

StatementService

• Removed redundant sort specification from PageRequest.of() (already defined in repository method names)
• Moved Instant.now() outside batch processing loop to ensure consistent timestamps within a batch
• Simplified token validation and removed redundant exception handling

Checklist:

• Public methods are documented
• Public methods are tested
• New and existing tests pass when run locally
• There are no new warnings or errors

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

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• checkstyle.org
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