Bug 1: Delete printer was not actually deleting archives when
delete_archives=True (default). The if-condition was inverted, causing
archives to remain and potentially causing FK constraint issues.
Bug 2: SmartPlug.printer_id had unique=True constraint, preventing
multiple HA scripts from being linked to the same printer. Removed the
constraint to allow multiple plugs/scripts per printer (matching
feature/176 behavior).
Closes#214