Add due date and priority fields to Task model

[sonnymay]

Summary

Extend the Task model with due_date and priority fields. This demonstrates schema evolution, Alembic migrations, and thoughtful API versioning — all practical backend skills.

Acceptance criteria

• Task model gains optional due_date (ISO 8601 date) and priority (enum: low / medium / high, default medium)
• Pydantic request/response schemas updated — both fields are optional to keep existing clients compatible
• GET /tasks/?priority=high filter works alongside existing pagination params
• GET /tasks/?due_before=2025-12-31 filter returns tasks due on or before the given date
• Alembic migration script included (or SQLAlchemy create_all comment updated with field notes)
• Tests cover creating a task with and without the new fields, and the new filter params

Notes

Priority enum and nullable date column are a common pattern in production task/project management APIs.