feat(01-03): tenant signup API, service, and UI
- Add businessAddress and contactPhone fields to Tenant schema
- Create src/lib/tenant.ts with createTenant() function:
- Validates input, slugifies business name, hashes password (bcrypt 12)
- Prisma transaction creates Tenant + admin User atomically
- Custom EmailAlreadyExistsError for 409 Conflict responses
- Create POST /api/tenants/signup route returning 201/400/409/500
- Create /signup page with full form (business name, owner info, password, optional fields)
- Client-side validation: required fields, email format, password match
- Redirects to /login?registered=true on success
- Update /login page to show success banner when ?registered=true
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