Feature: add GET /tasks?completed=true filter support

[ascerra]

Description

The GET /tasks endpoint returns all tasks with no way to filter. As the task list grows, users need a way to see only completed or only pending tasks.

Proposed behavior

GET /tasks                → all tasks (current behavior)
GET /tasks?completed=true → only completed tasks
GET /tasks?completed=false → only pending tasks

Acceptance criteria

• Query parameter is optional (no breaking change)
• Invalid values (e.g. ?completed=banana) should return 400
• Filtering happens server-side, not just in response

[hmariset]

/fs-status