fix(ci): remove user limit in test environment

Integration tests collectively create 51+ users across 15 test files
without cleanup, exceeding the MAX_USERS=50 cap in vitest.config.ts.
This caused cascading 401 failures in rbac-matrix, escalation,
custom-roles, and security-auth-hardening tests.

Set MAX_USERS=0 (unlimited) to match the Docker production default.

vitest.config.ts

@@ -51,7 +51,7 @@ export default defineConfig({
       MAX_UPLOAD_SIZE_MB: "10",
       MAX_BATCH_SIZE: "10",
       RATE_LIMIT_PER_MIN: "10000",
-      MAX_USERS: "50",
+      MAX_USERS: "0",
       MAX_MEGAPIXELS: "100",
       CONCURRENT_JOBS: "3",
       FILE_MAX_AGE_HOURS: "1",