Add pagination and filtering to GET /tasks/

[sonnymay]

Summary

The current GET /tasks/ endpoint returns all tasks in a single response. Adding pagination and filtering makes this a more realistic, production-grade API.

Acceptance criteria

• GET /tasks/?skip=0&limit=20 — offset-based pagination with configurable defaults
• GET /tasks/?status=completed — filter by completion status
• GET /tasks/?q=keyword — optional keyword search on task title
• Response includes total, skip, limit, and items fields (envelope pattern)
• Pydantic response schema updated to reflect the paginated envelope
• Existing tests updated; new tests cover pagination edge cases (empty page, last page)

Notes

This mirrors real-world API patterns used in production FastAPI services and is a common interview topic.