Fix task bugs and add search filter

[karlaumann-eliza]

Summary:

• Fixed GET /tasks status filtering to use the incoming status query value.
• Added case-insensitive q search across task title and description.
• Persisted task completion by updating the task list and saving it.

Verification:

• Verified status=open and status=done filtering with curl.
• Verified task completion persists and completed_at is set.
• Verified q search works.
• Verified q search composes with status filtering.
• Confirmed final diff is scoped to app/service.py.

[karlaumann-eliza]

Final verification completed.

Commands checked:

• curl "http://127.0.0.1:8000/tasks?status=open"
• curl "http://127.0.0.1:8000/tasks?status=done"
• curl -X POST http://127.0.0.1:8000/tasks/3/complete
• curl http://127.0.0.1:8000/tasks/3
• curl "http://127.0.0.1:8000/tasks?q=launch"
• curl "http://127.0.0.1:8000/tasks?status=open&q=plan"

Results:

• Status filtering is correct.
• Completion persists and sets completed_at.
• Search is case-insensitive.
• Search and status filters compose correctly.
• Final diff is reviewable and scoped to app/service.py.
• data/tasks.json runtime changes were restored before commit.

Final branch:
codex/lab-fixes

Commit:
229b11f Fix task bugs and add search filter