fix(syslog): a device through Docker's address stays one source when that address changes (#6717)

Docker's ingress address depends on the node Steward's task is on, and sources
were keyed by (address, sender). After a task move the same device arrived under
a new key and became a new "offered" row, so its lines were counted, not kept.

A named sender is now one row whatever address it last came from, enforced by a
partial unique index. Migration syslog_005 folds rows sharing a name into the
added one and moves their lines and patterns across.

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

docs/plugins/syslog.md

@@ -56,7 +56,9 @@ network, so the address no longer tells the devices apart. For such an address
 Steward keys the source by the hostname each line carries instead (#6096): each
 device is its own source. A device must put its hostname in its lines (NASes
 and routers do); lines with none at Docker's address all fold into one source,
-and the page says so.
+and the page says so. Docker's address changes when Steward's task moves to
+another node; a named source stays the same source, added or not, and takes the
+new address.
 
 Such a source's own address, which places its lines on its SNMP device's and
 host's pages (chart marks, the lines around an anomaly), is found by name: the