Backups: a revision chain belongs to a PC, not just a machine

CHANGELOG.md

@@ -21,6 +21,17 @@ ADR-007 and ADR-002.
   number now identifies the machine only. A new PC takes its hostname as its
   asset number, which is what the existing data already does, and an existing
   PC's asset number is never overwritten.
+- Configuration backups piled up duplicate revisions on any machine number
+  reported by more than one PC. A posted config was compared against the latest
+  revision for the machine, so where several PCs share a number - the part
+  markers do, and their configs differ by COM port - each PC's post differed
+  from whichever PC posted last, nothing deduped, and the table grew by a row
+  per PC per collection cycle. A backup chain is now per source PC, so an
+  unchanged config is a no-op again and each PC keeps its own history against
+  the machine it belongs to. Retention prunes each chain separately, so a busy
+  PC's revisions can no longer push out a quiet PC's only backup, and the
+  revision diff compares against the same PC's previous revision rather than
+  another PC's.
 - A backup export containing a value with an empty right-hand side (`Name=`)
   failed to parse, and with it the whole file, so that machine could never be
   backed up. The form is not strictly legal but occurs in real exports. It is