Fix task bugs and add search filter

[mouadel-hachimi-sw]

Summary

• Fixed status filtering for GET /tasks.
• Made task completion persist changes to data/tasks.json.
• Added case-insensitive q search across task titles and descriptions.
• Preserved existing behavior when q is omitted.
• Verified that data/tasks.json contains no test-only changes.

Verification

• GET /tasks?status=open returns only open tasks.
• GET /tasks?status=done returns only completed tasks.
• POST /tasks/1/complete followed by GET /tasks/1 confirms the completed status persists.
• GET /tasks?q=LAUNCH verifies case-insensitive title matching.
• GET /tasks?q=OAUTH verifies description matching.
• GET /tasks?status=open&q=plan verifies combined filtering.
• GET /tasks still returns all tasks when q is omitted.
• Python compilation and git diff --check pass.

Files Changed

• app/service.py