Fix task bugs and add search filter

[dmaiorano-reyit]

Summary

• Fix status filtering for GET /tasks?status=...
• Persist task completion updates and completed_at values
• Add case-insensitive q search across task title and description
• Add focused API tests for filtering, search composition, and completion persistence

Verification

• .venv/bin/pytest -q
• 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"