[BACKEND] Validate task status filter values on list endpoint

[utksh1]

Problem

GET /api/v1/tasks?status=... accepts arbitrary status strings and returns an empty result for invalid values. This makes client bugs harder to diagnose and weakens the API contract.

Expected behavior

The endpoint should validate status against the known TaskStatus values and return a structured 400 error for invalid values.

Acceptance criteria

• Validate the optional status query parameter before running the SQL query.
• Return HTTP 400 with a clear message for invalid status values.
• Add tests for valid and invalid status filters.

[HitanshiThakar]

Hi @utksh1, I’d like to work on this issue under GSSoC'26.
I recently contributed to this project and am already familiar with the task routes/tests. Planning to add validation against the existing TaskStatus values, return a structured 400 response for invalid filters, and add coverage for both valid and invalid status query cases.
Kindly assign it to me.