feat(syslog): a syslog receiver: UDP and TCP, senders offered before they're kept, each source's lines in a ring (#5999)

- plugins/syslog: RFC 5424 and RFC 3164 parsing, RFC 6587 TCP framing.
- Lines are queued as they arrive and stored every 5 s through a Holding,
  so a storage stall delays them rather than losing them.
- A new sender is an offered source, counted and not kept, until an admin
  adds it on the Syslog page (name, host); it can be ignored instead.
- The listener follows the plugin's settings (port, UDP, TCP), rebinding
  within seconds of a save; a port it can't open is said on the page.
- Hourly ring per source: age window and byte cap from Settings.

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

steward/settings/routes.py

@@ -975,7 +975,7 @@ def _build_plugin_cfg_from_form(plugin: dict, form) -> dict:
 # of a host), not discrete vendor integrations. Presentation-only split — they
 # still load through the normal plugin mechanism. A plugin.yaml may set
 # kind: capability|integration to override.
-CAPABILITY_PLUGINS = {"host_agent", "snmp", "docker"}
+CAPABILITY_PLUGINS = {"host_agent", "snmp", "docker", "syslog"}
 
 
 @settings_bp.get("/plugins/")