Multiple printers, printer/slot in Spoolman, kept in sync

- One worker per printer (name, IP, CC2 Control URL, own pinned host key);
  a spool sits in at most one slot of one printer, moves between printers
- Spoolman: location "<printer> Slot N" plus extra fields "drucker"/"slot"
  (created by the bridge), cleared and "Lager" on removal
- Full comparison every minute: the printer wins over manual edits of loaded
  spools; stale printer locations go to storage; own locations untouched
- Automatic migration of v1 settings, state and host key
- Status hint when a tag is still in a tray but unmapped (e.g. after
  "Take out" in CC2 Control)
- SSH test: connection drops during exec give a clean error, not a 500
- Mock environment with several printers; tests for all of the above

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014GcEk5tkfFSYD6NTF66NkC

.env.example

@@ -1,4 +1,6 @@
 # Startwerte. Nach dem ersten Start gelten die Einstellungen aus der Oberfläche (/data/settings.json).
+# Erster Drucker; weitere in der Oberfläche unter Einstellungen → Drucker.
+PRINTER_NAME=Centauri Carbon 2 Rechts
 PRINTER_HOST=10.10.2.202
 SSH_PORT=22
 SSH_USER=root