Printer assignment end to end, and a container that keeps a site's files

Assign printers to a MACHINE and let whichever PC controls it install them, with
the observed half kept strictly apart from the assigned half: a collector report
can never write an assignment row. Adoption is explicit. Adds a singular
relationship type so defaultprinter can mean exactly one, client scripts that
converge a bay's queues and correct a drifted one in place, and a wave gate so a
fleet-wide driver rollout cannot stampede its own share.

On the container side: the compose stack never persisted the instance directory,
so a rebuild discarded plugins.json and every upload. It is a volume now, the
image ships instance/ owned by the app user so the mountpoint is writable, and
the database moves to MySQL 8.4 LTS, which the Windows runbook has recommended
since 8.0 went end of life. The image also could not build at all: the frontend
stage never copied the plugin staging script it runs.

Adds a topology migration guide covering IIS to Docker and back.

frontend/src/api/index.js

@@ -306,6 +306,25 @@ export const printersApi = {
   list(params = {}) {
     return api.get('/printers', { params })
   },
+
+  // Printer assignment for one asset - a MACHINE normally, since the assignment
+  // belongs to the bay and reaches whichever PC controls it. Reads and writes
+  // that asset's OWN rows: the resolved view (own, else inherited) is
+  // /printers/for-host, which the client uses, not the editor.
+  assignment: {
+    get(assetid) {
+      return api.get(`/printers/assignments/for-asset/${assetid}`)
+    },
+    // The WHOLE set in one call. A row-at-a-time edit is a non-atomic
+    // reconcile: an HTTP failure part way leaves an asset half-assigned, with
+    // nothing recording what was meant.
+    set(assetid, printerassetids, defaultprinterassetid) {
+      return api.put(`/printers/assignments/for-asset/${assetid}`, {
+        printerassetids,
+        defaultprinterassetid: defaultprinterassetid ?? null
+      })
+    }
+  },
   // Every printer, paged past the backend's 100-row cap. Batch label printing
   // and "pick any record" dropdowns must use this: list() with a large
   // perpage is clamped to 100 and still returns a success response, so