feat: reduce Community Edition user limit from 50 to 10

Reduces the hardcoded max users for Community Edition from 50 to 10.

Changes:
- Backend: MAX_USERS_COMMUNITY constant (enforcement)
- Frontend: Display messages on users and tenants pages
- README: Documentation update

Ref: GT-Edge-AI-Internal/GT-2.0#289

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/control-panel-frontend/src/app/tenants/page.tsx

@@ -143,7 +143,7 @@ export default function TenantsPage() {
         domain: 'betatest',
         template: 'development',
         status: 'suspended',
-        max_users: 50,
+        max_users: 10,
         current_users: 12,
         namespace: 'gt-tenant-betatest',
         resource_count: 5,