Database setup check: warning raised by informational rules and a Slave_lag false positive #1192

Nextcloud 35.0.1, serverinfo 7.0.0, MariaDB 10.11 (Ubuntu 24.04), standalone server.

1. DatabaseChecks::run() returns SetupResult::warning() whenever
   $summary['failing'] > 0, regardless of rule severity. Rules flagged as
   "Information" (Sort_rows, Handler_read_first_rate, Handler_read_rnd_rate,
   Handler_read_rnd_next_rate…) therefore produce a permanent admin warning.
   Their thresholds ("aim for less than 1 per hour") are unreachable for any
   real Nextcloud workload, and the queries come from Nextcloud core itself.
   Suggestion: only raise a warning for SEVERITY_ALERT (and maybe WARNING),
   report notices as info, or allow admins to acknowledge/mute individual rules.

2. Rule Slave_lag (RuleSet/Mysql.php): formula 'Slave_running', test 'value == 0',
   requires ['Slave_running']. Slave_running is always present (OFF) on a
   standalone server, so the rule always fails ("A configured replica is not
   running") although SHOW REPLICA STATUS is empty and log_bin is OFF.
   Suggestion: skip the rule when no replica is configured.