Add JWT authentication with bearer token support

[sonnymay]

Summary

The API currently has no authentication layer. Adding JWT bearer token auth would make this a realistic production-quality service and demonstrate a key backend skill.

Acceptance criteria

• POST /auth/register — create a user (email + hashed password via passlib)
• POST /auth/token — return a signed JWT on valid credentials
• Protected routes require a valid Authorization: Bearer <token> header
• Invalid or expired tokens return 401 Unauthorized with a consistent error shape
• Auth logic lives in app/auth/ (router, schemas, utils) — not mixed into task routes
• Existing pytest suite still passes; add test_auth.py covering register, login, and rejection cases

Stack

python-jose or PyJWT for token signing · passlib[bcrypt] for password hashing · FastAPI Depends for route protection