feat(02-02): user CRUD API + admin page, tenant CRUD API + admin page

- Create UserController with GET/POST/PATCH/DELETE endpoints at /users
  - ADMIN sees own-tenant users only; SUPER_ADMIN sees all (T-02-10)
  - ADMIN cannot escalate to SUPER_ADMIN role (T-02-08)
  - ADMIN cannot delete self or cross-tenant users
- Create TenantController with GET/POST/PATCH/DELETE at /tenants
  - SUPER_ADMIN-only access (D-10)
  - Tenant deletion blocked if active users exist (T-02-09)
- Create CreateUserDto, UpdateUserDto, CreateTenantDto with class-validator
- Create admin/users page with user table, create/edit/delete modals
- Create admin/tenants page with tenant table, create/edit/deactivate (SUPER_ADMIN only)
- Add admin section to sidebar: Verwaltung > Benutzer + Mandanten
  - Verwaltung visible for ADMIN/SUPER_ADMIN; Tenants link SUPER_ADMIN only
- Install @nestjs/mapped-types for PartialType DTO pattern

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/package.json

@@ -13,6 +13,7 @@
     "@nestjs/config": "^4.0.0",
     "@nestjs/core": "^11.0.0",
     "@nestjs/jwt": "^11.0.2",
+    "@nestjs/mapped-types": "^2.1.1",
     "@nestjs/passport": "^11.0.5",
     "@nestjs/platform-express": "^11.0.0",
     "@prisma/client": "^6.0.0",