[BE-46] Implement global exception filter and response interceptor

[yusuftomilola]

Overview

Implement a custom exception filter and response interceptor to standardise all API response shapes inside backend/contrib/.

Location

All work must be inside backend/contrib/src/common/.

Acceptance Criteria

• HttpExceptionFilter catches all HttpException instances and returns { statusCode, message, error, timestamp, path }
• TransformInterceptor wraps all successful responses as { success: true, data: <payload>, timestamp }
• Both applied globally in AppModule via APP_FILTER and APP_INTERCEPTOR
• Existing endpoints continue to pass their responses through the interceptor
• Swagger docs updated to reflect the wrapper shape on at least one controller

[akargi]

@akargi has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I am writing to request that the issue be assigned to me for resolution.
I am confident in my ability to handle and resolve it effectively.
Please grant me access or ownership so I can begin work immediately.
Let me know if there are any prerequisites or additional details required.
Thank you for your consideration.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @akargi to this issue.

[drips-wave]

Congratulations, @akargi! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @akargi: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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