Refactor/standardize api responses

[Adamsonoladipupo]

Summary

Standardized API JSON responses across the backend controllers using a reusable response wrapper.

Changes

• Added a centralized response wrapper utility.
• Refactored existing controllers to use the standardized response format.
• Standardized successful and error responses.
• Preserved existing HTTP status codes and business logic.
• Added/updated tests for the standardized response structure.
• Maintained the existing Controller → Service → Model architecture.

Standard Response Format

{
  "success": true,
  "data": {},
  "error": null,
  "message": "Operation successful"
}

Error responses follow the same structure with success: false.

Verification

• Tests: Passed
• Type checking: Passed
• Linting: Passed
• Build: Passed

Proof of Work

API responses were verified locally and confirmed to follow the standardized response structure.

Closes #105

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