serverinfo "Replica lag" check (Slave_running) always fails on standalone (non-replica) MySQL/MariaDB servers #1170

Describe the bug

The database health check introduced in serverinfo (Settings → Administration → Overview → Database checks) always reports the "Replica lag" rule (Slave_lag, formula Slave_running, test value == 0) as failed on a standalone database server that was never configured for replication.

Slave_running is a MySQL/MariaDB global status variable that is OFF on ANY server not acting as a replica — this is the normal, expected state, not an error. The rule doesn't check whether the server is actually configured as a replica (e.g. via SHOW REPLICA STATUS returning a row, or read_only/gtid_slave_pos being set) before evaluating.

Source: apps/serverinfo/lib/Database/RuleSet/Mysql.php

$rules[] = new Rule(
    id: 'Slave_lag',
    name: 'Replica lag',
    category: 'Replication',
    severity: Rule::SEVERITY_ALERT,
    formula: 'Slave_running',
    test: 'value == 0',
    issue: 'A configured replica is not running.',
    recommendation: 'Check the replica with SHOW REPLICA STATUS; resolve any IO/SQL errors and restart replication.',
    justification: 'Slave_running is reporting OFF.',
    justificationFormula: '',
    requires: ['Slave_running'],
);

To Reproduce

1. Run any standalone Nextcloud instance with a single, non-replicated MySQL/MariaDB server.
2. Open Settings → Administration → Overview → Database checks.
3. "Replica lag" is listed under "Failed" with "A configured replica is not running." — even though no replica was ever configured.

Expected behavior

The rule should first verify that the instance is actually configured as a replica (e.g. SHOW REPLICA STATUS returns a row) before evaluating Slave_running, and be skipped entirely otherwise — consistent with how other inapplicable rules in the same rule set are already skipped ("Skipped").

Environment

• Nextcloud 35.0.1 (serverinfo 7.0.0) — first observed immediately after upgrading 33.0.9 → 34.0.4 → 35.0.1, so likely introduced in serverinfo 6.0.0 or 7.0.0
• MariaDB 10.11.14
• Confirmed via SHOW REPLICA STATUS returning zero rows (no replica configured at all)