feat: Add Repository pattern for Vault entity (#633)

[raphael-01-visual]

Description

Resolves #633
Related #362

What:
Extracts all direct TypeORM data access calls from VaultsService into a newly created, dedicated VaultRepository.

Why:
Previously, VaultsService was tightly coupled with TypeORM's generic Repository<Vault>. By separating the data access layer from the business logic layer, this refactoring significantly improves the testability, maintainability, and modularity of the Vaults domain.

Changes Made

• Created VaultRepository: Added backend/src/vaults/vault.repository.ts as an abstraction layer with explicit data-access methods (findAll, findById, save, and findLeaderboard).
• Refactored VaultsService: Replaced all TypeORM Repository<Vault> usages with calls to the new VaultRepository.
• Updated VaultsModule: Injected and exported the VaultRepository provider where necessary.
• Updated Tests: Adapted the VaultsService unit tests (vaults.service.spec.ts) to successfully mock and test against the new custom repository methods, ensuring 100% functionality retention.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves code structure/readability)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have updated the unit tests to pass cleanly

Closes #362
Closes #361
Closes #359
Closes #360

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[raphael-01-visual]

@vic-Gray , pls review PR