Fix task bugs and add search filter

[vedantsahai]

Summary

• Fix GET /tasks status filtering so the handler compares against the requested status value instead of the literal string "status".
• Persist task completions by mutating the stored task, stamping completed_at, and saving via save_tasks, ensuring follow-up reads show the done state.
• Add case-insensitive q search across task title and description, and make sure it composes cleanly with the status filter.

Testing

• curl "http://127.0.0.1:8000/tasks?status=open" → returns only open tasks.
• curl "http://127.0.0.1:8000/tasks?status=done" → returns only done tasks.
• curl -X POST http://127.0.0.1:8000/tasks/3/complete → returns task 3 with status: done and a populated completed_at.
• curl http://127.0.0.1:8000/tasks/3 → task 3 remains done with the same timestamp.
• curl "http://127.0.0.1:8000/tasks?q=launch" → returns task 1.
• curl "http://127.0.0.1:8000/tasks?status=done&q=plan" → returns task 3, demonstrating combined filters work.