Fix task bugs and add search filter

[kanishk-aivar]

Summary

• Fix GET /tasks status filtering so it matches the query value instead of the literal "status".
• Persist task completion by writing the updated record back through save_tasks().
• Add case-insensitive q search across title and description; it works with the status filter and is a no-op when omitted.

Test plan

• 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 then GET /tasks/3 shows "status": "done" and a non-null completed_at
• curl "http://127.0.0.1:8000/tasks?q=launch" matches title/description
• curl "http://127.0.0.1:8000/tasks?status=open&q=plan" combines filters
• Confirm data/tasks.json was not committed as test churn