feat: plugin extension slots for access, request interceptors, Users actions, and form fields

Four purely-additive seams that let build-time tooling extend host
behaviour without forking files:

* `src/access.extensions.ts` — identity `applyAccessExtensions`
  (mirrors `config/routes.extensions.ts`). `src/access.ts` runs the
  resolved predicate object through it. Adds two new predicates,
  `canSeeOrgAdmin` and `canManageCurrentOrg`, alongside the existing
  `canSeeAdmin`. Resources, Models children, Cluster Management, and
  Resources/Backends are retagged from `canSeeAdmin` to
  `canSeeOrgAdmin`. Users and Dashboard stay strict.
* `src/request.extensions.ts` — identity-empty
  `extraRequestInterceptors`. `src/request-config.tsx` spreads it
  into the existing `requestInterceptors` list so extensions can
  inject context-aware headers without forking the request config.
* Users page action column — renders
  `getGPUStackPlugin()?.components?.UserRowActions` next to the
  existing DropdownButtons inside a Space when a plugin component
  is registered. If absent, the cell renders exactly as before.
* `src/components/plugin-extra-fields.tsx` — generic component-slot
  helper. Renders `pluginManager.components.<name>` if registered,
  forwarding a `context` payload. Used by create/edit forms to let
  plugins inject extra `Form.Item` fields. Mounted in the relevant
  create forms — API Keys, Cloud Credentials, Clusters, Model
  Routes, Model Providers, and Inference Backends — under the slot
  name `CreateOrgScopeField`. Resources whose org is implicit from a
  parent (Models / Workers / Benchmarks / Worker Pools / Model Files
  inherit from the chosen Cluster) deliberately don't mount the slot.

src/pages/backends/components/backend-card.tsx

@@ -1,3 +1,4 @@
+import PluginExtraFields from '@/components/plugin-extra-fields';
 import {
   AutoTooltip,
   DropdownActions,
@@ -248,30 +249,39 @@ const BackendCard: React.FC<BackendCardProps> = ({
     const source = data.is_built_in
       ? BackendSourceLabelMap[BackendSourceValueMap.BUILTIN] || ''
       : BackendSourceLabelMap[data.backend_source] || '';
+    const ownerTag = (
+      <PluginExtraFields
+        name="BackendOwnerTag"
+        context={{ organizationId: data.organization_id }}
+      />
+    );
     if (!source) {
-      return null;
+      return ownerTag;
     }
     return (
-      <Tag
-        color={
-          TagColorMap[
-            data.is_built_in
-              ? BackendSourceValueMap.BUILTIN
-              : data.backend_source
-          ]
-        }
-        className="font-400"
-        variant="filled"
-        style={{
-          borderRadius: 'var(--ant-border-radius)',
-          margin: 0,
-          width: 'max-content'
-        }}
-      >
-        {intl.formatMessage({
-          id: source
-        })}
-      </Tag>
+      <div style={{ display: 'flex', gap: 6, alignItems: 'center' }}>
+        <Tag
+          color={
+            TagColorMap[
+              data.is_built_in
+                ? BackendSourceValueMap.BUILTIN
+                : data.backend_source
+            ]
+          }
+          className="font-400"
+          variant="filled"
+          style={{
+            borderRadius: 'var(--ant-border-radius)',
+            margin: 0,
+            width: 'max-content'
+          }}
+        >
+          {intl.formatMessage({
+            id: source
+          })}
+        </Tag>
+        {ownerTag}
+      </div>
     );
   };
 

src/pages/backends/config/index.ts

@@ -109,7 +109,11 @@ export const backendActions = [
     icon: icons.DeleteOutlined,
     locale: true,
     danger: true,
-    show: (record: any) => !record.is_built_in
+    // Platform built-ins are admin-curated and not user-deletable.
+    // An org-scoped override of a built-in IS deletable — deleting
+    // the override is how the user reverts to the Platform row.
+    // Plain custom backends are always deletable.
+    show: (record: any) => !record.is_built_in || record.organization_id != null
   }
 ];
 

src/pages/backends/forms/basic.tsx

@@ -1,3 +1,4 @@
+import PluginExtraFields from '@/components/plugin-extra-fields';
 import { PageAction } from '@/config';
 import { backendOptionsMap } from '@/pages/llmodels/constants/backend-parameters';
 import {
@@ -67,6 +68,10 @@ const BasicForm = () => {
           required
         ></CInput.Input>
       </Form.Item>
+      <PluginExtraFields
+        name="CreateOrgScopeField"
+        context={{ action, allowGlobal: true }}
+      />
       <Form.Item<FormData> hidden name="backend_source">
         <CInput.Input></CInput.Input>
       </Form.Item>

src/pages/backends/index.tsx

@@ -166,7 +166,15 @@ const BackendList = () => {
     }
     // ================ Delete ================
     if (item.action === 'delete') {
-      if (item.data.backend_source === BackendSourceValueMap.COMMUNITY) {
+      // Platform community rows aren't actually deletable — the
+      // "delete" action there is a soft-disable on the admin-curated
+      // catalog. An org-scoped row of any source (including an org's
+      // override of a community backend) IS owner-mutable data and
+      // gets a real DELETE.
+      const isPlatformCommunity =
+        item.data.backend_source === BackendSourceValueMap.COMMUNITY &&
+        item.data.organization_id == null;
+      if (isPlatformCommunity) {
         modalRef.current?.show({
           content: 'backends.title',
           operation: 'common.delete.single.confirm',