Fix task filtering, search, and completion persistence

[jclasley]

Summary

• Make /tasks honor the status query value instead of comparing against a literal string, so status filters return expected results.
• Add case-insensitive q filtering across task titles and descriptions so clients can search tasks from query params.
• Persist completion updates in complete_task so status=done and completed_at survive subsequent reads.

Verification

• Confirmed curl \"http://127.0.0.1:8000/tasks?q=launch\", curl \"http://127.0.0.1:8000/tasks?q=report\", and curl \"http://127.0.0.1:8000/tasks?status=open&q=plan\" each return the expected records.
• Confirmed completing a task is persisted and re-fetching the task still shows status=done with non-null completed_at.
• Reviewed data/tasks.json; runtime test data was restored and is not included in this PR.