feat: add admin ideas management page with private notes

Adds a dedicated super admin page for managing idea submissions across
all tenants. Includes status summary cards, filterable/searchable table,
detail modal with status updates, and private admin notes for internal
tracking (sprint refs, thoughts, follow-ups). Notes are not visible to
tenant users.

- Database: admin_note column on shared.ideas (019 migration)
- Backend: PUT /admin/ideas/:id/note endpoint
- Frontend: AdminIdeasPage with table, filters, detail modal
- Sidebar: "Idea Submissions" nav link in admin sections
- Routing: /admin/ideas route under SuperAdminRoute guard

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/modules/auth/admin.controller.ts

@@ -218,6 +218,17 @@ export class AdminController {
     return { success: true, idea };
   }
 
+  @Put('ideas/:id/note')
+  async updateIdeaNote(
+    @Req() req: any,
+    @Param('id') id: string,
+    @Body() body: { adminNote: string },
+  ) {
+    await this.requireSuperadmin(req);
+    const idea = await this.ideasService.updateNote(id, body.adminNote);
+    return { success: true, idea };
+  }
+
   @Put('organizations/:id/settings')
   async updateOrgSettings(
     @Req() req: any,