fix(tenants): keep the 'system' username when provisioning a new tenant

Problem
Commit 3d3ce92e9 personalised the three users seeded into a brand-new
tenant database and renamed user id 2 from 'system' to
'<tenant>_system' (for example 'default_system'). Fineract looks this
user up by the fixed name AppUserConstants.SYSTEM_USER_NAME ('system')
through AppUserRepositoryWrapper.fetchSystemUser(), so on every fresh
tenant:
- every scheduler job failed once a minute with
  "UserNotFoundException: User with name 'system' does not exist"
  (seen with the Send Asynchronous Events job);
- COB initialisation, savings escheat, current account closure and the
  stuck-job listener would fail in the same way.
Existing tenants were not affected because the provisioner only runs
right after the very first migration of a new tenant.

Fix
- TenantInitialUserProvisioner now sets user id 2's username to
  AppUserConstants.SYSTEM_USER_NAME and only replaces its password.
- Admin and interop users are still personalised as
  '<tenant>_admin' and '<tenant>_interop'.
- Javadoc and the startup log line now say the system user keeps its
  name and why.

Resulting users on a new tenant
- <tenant>_admin    (password: initial-user-password or built-in default)
- system            (same password, name unchanged)
- <tenant>_interop  (same password)

Tenants already created with '<tenant>_system' need a one-off fix:
  UPDATE m_appuser SET username = 'system' WHERE id = 2;

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

fineract-provider/src/main/java/org/apache/fineract/infrastructure/core/service/migration/TenantInitialUserProvisioner.java

@@ -22,6 +22,7 @@ import java.util.Locale;
 import javax.sql.DataSource;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
+import org.apache.fineract.useradministration.service.AppUserConstants;
 import org.springframework.core.env.Environment;
 import org.springframework.jdbc.core.JdbcTemplate;
 import org.springframework.security.crypto.factory.PasswordEncoderFactories;
@@ -37,8 +38,9 @@ import org.springframework.stereotype.Component;
  * touched. The seeded changeSet itself is deliberately left alone: editing it would change its Liquibase checksum and
  * break validation on every tenant that already ran it.
  *
- * Usernames become {@code <tenantIdentifier>_admin}, {@code <tenantIdentifier>_system} and
- * {@code <tenantIdentifier>_interop}. All three share one password: the value of
+ * Usernames become {@code <tenantIdentifier>_admin} and {@code <tenantIdentifier>_interop}. The system user keeps the
+ * {@link AppUserConstants#SYSTEM_USER_NAME} username because scheduler jobs, COB and other background processes look it
+ * up by that name; only its password is changed. All three share one password: the value of
  * {@code fineract.tenant.initial-user-password} (env {@code FINERACT_TENANT_INITIAL_USER_PASSWORD}) when set,
  * otherwise {@link #DEFAULT_PASSWORD}.
  */
@@ -68,11 +70,11 @@ public class TenantInitialUserProvisioner {
         final String encoded = encoder.encode(password);
 
         update(jdbc, ADMIN_ID, prefix + "_admin", encoded);
-        update(jdbc, SYSTEM_ID, prefix + "_system", encoded);
+        update(jdbc, SYSTEM_ID, AppUserConstants.SYSTEM_USER_NAME, encoded);
         update(jdbc, INTEROP_ID, prefix + "_interop", encoded);
 
-        log.warn("Tenant '{}' initial users created: {}_admin, {}_system, {}_interop (password: {}). Change it after first login.",
-                tenantIdentifier, prefix, prefix, prefix, overridden ? "from " + PASSWORD_PROPERTY : "built-in default");
+        log.warn("Tenant '{}' initial users created: {}_admin, {}, {}_interop (password: {}). Change it after first login.",
+                tenantIdentifier, prefix, AppUserConstants.SYSTEM_USER_NAME, prefix, overridden ? "from " + PASSWORD_PROPERTY : "built-in default");
     }
 
     private void update(final JdbcTemplate jdbc, final long id, final String username, final String encodedPassword) {