One printer picker for machines and PCs, and one default per asset

The assignment belongs to the MACHINE, and until now there was no way to set it
except the generic relationships card or the API - the form for the thing the
feature is about did not exist. MachineForm now carries the picker, and PCForm
uses the SAME component rather than its own copy: the PC's set overrides the
machine's, and two implementations of that would drift, with the two ends of an
override disagreeing being exactly the bug nobody would spot.

The shared picker also fixes what PCForm did on save. It wrote row at a time
through the generic relationship endpoints, which is a non-atomic reconcile: an
HTTP failure part way left a PC half-assigned with nothing recording what was
meant. It now calls the reconcile endpoint, which validates the default before
writing anything.

A relationship type can now say it allows one active row per asset
(relationshiptypes.issingular, migration 7d34), and defaultprinter says it.
Cardinality belongs to the type rather than the printers plugin: core's create
path is where every hand-made link passes, and the next type meaning "exactly
one" gets the rule for free. Setting a second default REPLACES the first instead
of refusing, because "make this the default" means that - and a card answering
409 would leave the user hunting for the old row.

Without it the schema was happy to hold two defaults: the unique constraint is
(source, target, type), so two different targets are two valid rows, and the
resolver takes the OLDEST - the new default silently lost. Proven by disabling
the new rule and watching the tests fail.

FOUND WHILE TESTING IN A BROWSER, and it was not mine: MachineForm read
.data.data off computersApi.listAll(), which resolves to the ARRAY - fetchAllPages
has already unwrapped every page. The whole parallel load threw into the catch,
so every dropdown on the machine edit form came up empty and the machine's own
values never loaded. A build cannot see this; only opening the page can.

GET /api/printers/assignments/for-asset/<id> returns an asset's OWN assignment,
without inheritance, because the editor must show what this asset's rows say -
otherwise a machine's printers appear ticked on the PC that inherits them and
unticking one silently creates an override.

docs/api-inventory.json

@@ -3047,6 +3047,14 @@
         "params": "none",
         "purpose": "Dashboard card: printers needing a cartridge, one row per printer. Reuses the low-supplies query and its five-minute cache, so the card costs the same as the report",
         "example": "curl -H \"Authorization: Bearer $TOKEN\" http://localhost:5001/api/printers/dashboard/supplies"
+      },
+      {
+        "method": "GET",
+        "path": "/api/printers/assignments/for-asset/<asset_id>",
+        "auth": "optional jwt",
+        "params": "asset_id in path",
+        "purpose": "This asset's OWN printer assignment - printerassetids and defaultprinterassetid - deliberately WITHOUT inheritance. The editor has to show what this asset's own rows say, or a machine's printers would appear ticked on the PC that inherits them and unticking one would silently create an override. /printers/for-host is the resolved view the client uses; this is the editable one",
+        "example": "curl http://localhost:5001/api/printers/assignments/for-asset/14"
       }
     ]
   },

docs/openapi.json

@@ -2,7 +2,7 @@
  "openapi": "3.1.0",
  "info": {
   "title": "ShopDB Flask API",
-  "version": "0.11.2",
+  "version": "0.11.3",
   "description": "Asset-management API (core + plugins). Responses use a `success_response` envelope: `{status, data, meta}`. Auth: Bearer JWT (login or a managed PAT) for `jwt`/`admin`/`permission:*`; `X-API-Key` for collector/managed-token endpoints; public endpoints need neither."
  },
  "servers": [
@@ -16862,6 +16862,50 @@
       }
      }
     }
+   },
+   "get": {
+    "tags": [
+     "plugin-printers"
+    ],
+    "summary": "This asset's OWN printer assignment - printerassetids and defaultprinterassetid - deliberately WITHOUT inheritance. The...",
+    "description": "This asset's OWN printer assignment - printerassetids and defaultprinterassetid - deliberately WITHOUT inheritance. The editor has to show what this asset's own rows say, or a machine's printers would appear ticked on the PC that inherits them and unticking one would silently create an override. /printers/for-host is the resolved view the client uses; this is the editable one\n\n**Auth:** optional jwt\n\n**Params:** asset_id in path\n\n**Example:**\n```\ncurl http://localhost:5001/api/printers/assignments/for-asset/14\n```",
+    "security": [
+     {},
+     {
+      "bearerAuth": []
+     }
+    ],
+    "responses": {
+     "200": {
+      "description": "Success. Body is the success_response envelope: {status, data, meta}.",
+      "content": {
+       "application/json": {
+        "$ref": "#/components/schemas/SuccessEnvelope"
+       }
+      }
+     },
+     "default": {
+      "description": "Error. Body is the error envelope; the code and message are nested under data.error.",
+      "content": {
+       "application/json": {
+        "$ref": "#/components/schemas/ErrorEnvelope"
+       }
+      }
+     },
+     "404": {
+      "description": "No such record."
+     }
+    },
+    "parameters": [
+     {
+      "name": "asset_id",
+      "in": "path",
+      "required": true,
+      "schema": {
+       "type": "string"
+      }
+     }
+    ]
    }
   },
   "/api/printers/{printer_id}": {