Add tests, fix bugs, and implement assign endpoint

[P-kaizoku]

What's included

• Tests — 85 tests (unit + integration) with 97%+ coverage
• Bug fixes — Fixed 3 bugs found through testing:
  • getByStatus doing substring match instead of exact match
  • getPaginated off-by-one error (page 1 was skipping results)
  • completeTask silently resetting priority to "medium"
• New feature — PATCH /tasks/:id/assign with input validation
• Bug report — See BUG_REPORT.md
• Submission notes — See SUBMISSION_NOTES.md

How to run

cd task-api
npm install
npm test
npm run coverage