feat: Sprint 0 — Django API scaffold (users, stores, categories, JWT auth)

[leimo2011]

Sprint 0 — Django API Foundation

Closes #T-004, #US-001, #US-002, #US-003, #US-010, #US-011, #US-043

What's included

• ✅ Django 5 + Django REST Framework
• ✅ Custom User model (email login, roles: USER / ADMIN / SUPERADMIN)
• ✅ JWT Authentication (register, login, token refresh, profile)
• ✅ Store model + StoreLink model
• ✅ Category model
• ✅ Swagger API docs at /api/docs/
• ✅ Settings split: base / local / production
• ✅ CORS configured for React (localhost:3000, localhost:5173)

API Endpoints

Method	Endpoint	Description
POST	/api/auth/register/	Register new user
POST	/api/auth/login/	Login, get JWT tokens
POST	/api/auth/token/refresh/	Refresh access token
GET/PATCH	/api/auth/profile/	Get/update profile
GET/POST	/api/stores/	List / create stores
PATCH	/api/stores/{id}/live-status/	Toggle live status
GET/POST	/api/stores/links/	Manage store links
GET	/api/categories/	List categories
GET	/api/docs/	Swagger UI