Fix task bugs and add search filter

[krishnad2000]

Summary

• fix the GET /tasks status filter to use the provided status value
• persist task completion through the JSON storage layer
• add case-insensitive q filtering across title and description
• keep data/tasks.json restored so the PR only contains code changes

Verification

• GET /tasks?status=open returns only open tasks
• GET /tasks?status=done returns only done tasks
• POST /tasks/1/complete followed by GET /tasks/1 shows persistence
• GET /tasks?q=WORKSHOP, GET /tasks?q=oauth, and GET /tasks?status=open&q=launch return the expected matches