Add collector PC->printer links and searchable custom fields

Collector: the computers collector schema gains defaultprinter and
printers; apply_collector_payload resolves each reported identifier to
a printer asset (windowsname/hostname/sharename/assetnumber/IP,
first-hit case-insensitive) and idempotently syncs relationships -
defaultprinter (directional) for the default, connectedto for the
rest. Collector-created rows are tagged so a re-report archives dropped
links while manual relationships are never touched; unresolved
identifiers warn instead of failing. Both PC and printer detail pages
show the links via the shared relationships card (no frontend change).
GE-Enforce Win32_Printer collection snippet documented.

Searchable custom fields: a per-field searchable flag (migration 7d24);
global search matches custom-field values on flagged active fields and
routes each hit to the asset detail page, reusing the existing
(type,id) dedupe and search_<type>_enabled domain filter. Searchable
toggle on the Custom Fields settings page.

822 tests pass; both verified live.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/CONFIG.md

@@ -319,6 +319,15 @@ One boolean key per search domain, keyed `search_<type>_enabled` (default
 `true`). Toggles whether a domain appears in global search results. The set is
 generated from `SEARCH_DOMAINS` in `shopdb/core/api/settings.py`.
 
+## Custom fields
+
+Site-defined extra attributes per asset type (Settings > Custom Fields, table
+`customfields`). Each field has a `searchable` flag (default off). When on, the
+field's stored values are matched by global search and a hit routes to the
+owning asset's detail page. The asset's `search_<type>_enabled` domain toggle
+still applies, so a custom-field hit on a computer only shows when the computer
+search domain is enabled. Inactive or non-searchable fields are never matched.
+
 ---
 
 ## See also