feat(syslog): pattern alerts: telling phrases counted per source each minute for alert rules, storage-trouble presets (#6001)

A pattern (phrases, a severity, a source, a window) is counted every minute
into plugin_metrics as lines_matching_<key> per added source, zero included,
so an ordinary alert rule fires on a match and clears once the window passes.
Each count reads only the lines stored since the last, by line id; a new or
changed pattern reads its window once.

Patterns page with ready-made storage-trouble presets and "Alert on it",
which opens the rule form aimed at the count. Source names are kept unique,
and a rename moves its counts and the rules on them.

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

steward/core/formatting.py

@@ -376,7 +376,7 @@ SOURCES: dict[str, str] = {
     "icmp": "Ping monitors", "tcp": "TCP monitors", "dns": "DNS monitors",
     "http": "HTTP monitors", "steward": "Steward itself", "host_agent": "Host agent",
     "docker": "Docker containers", "traefik": "Traefik routers", "unifi": "UniFi",
-    "snmp": "SNMP devices", "prometheus": "Prometheus targets",
+    "snmp": "SNMP devices", "prometheus": "Prometheus targets", "syslog": "Syslog patterns",
 }