Fix task filters and completion persistence

[ArunJanardhanan-NTT]

Summary

• Fix GET /tasks status filtering to compare against the requested status value
• Add case-insensitive q search across task title and description
• Persist completed tasks by saving the updated task list

Validation

• Verified GET /tasks, GET /tasks?status=open, and GET /tasks?status=done with curl
• Verified POST /tasks/1/complete persists with a fresh GET /tasks/1 and JSON storage check
• Verified q search with curl for title, description, no-match, and status+q cases