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>

plugins/syslog/plugin.yaml

@@ -0,0 +1,20 @@
+# plugins/syslog/plugin.yaml
+name: syslog
+version: "1.0.0"
+description: "Receives syslog from NASes, routers and other devices, and keeps the lines of the sources you add."
+author: "Steward"
+license: "MIT"
+min_app_version: "0.1.0"
+tags:
+  - syslog
+  - logs
+
+# Sources are added from the Syslog page (/plugins/syslog/), where a device
+# that starts sending is offered. Steward runs as an unprivileged user, so
+# the port is above 1024: point devices at it, or map 514 to it.
+config:
+  port: 5514
+  udp: true
+  tcp: true
+  retention_days: 14
+  max_bytes_per_source: 50000000