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

prisma/schema.prisma

@@ -52,6 +52,11 @@ model Tenant {
   slug      String       @unique
   ownerEmail String
 
+  /// Physical address of the ISP business (optional)
+  businessAddress String?
+  /// Primary contact phone number (optional)
+  contactPhone    String?
+
   status             TenantStatus @default(ACTIVE)
   /// Timestamp when suspension was triggered (starts grace period clock)
   suspendedAt        DateTime?