feat(html-embed): per-workspace feature toggle, default OFF

The admin-only raw HTML/JS embed is a deliberate stored-XSS surface, so gate the
whole feature behind a workspace toggle that is OFF by default; it only works
when a workspace admin explicitly enables it.

- settings.htmlEmbed (boolean, default false) + workspace-update field htmlEmbed,
  persisted via WorkspaceRepo.updateSetting with an audit diff. Flipping it is
  admin-only (same Manage Settings CASL as other workspace toggles).
- New gate htmlEmbedAllowed(featureEnabled, role) = featureEnabled && admin/owner.
  All 7 server write paths (create, duplicate, collab onStoreDocument, REST/MCP/AI
  updatePageContent, single + zip import, transclusion unsync) now read the
  workspace's settings.htmlEmbed and strip unless (toggle ON AND admin). OFF
  (default, or a failed/empty workspace lookup) strips htmlEmbed for EVERYONE
  including admins -> existing embeds are cleaned up on next save, none persist.
- Client (defense-in-depth): the /html slash item is hidden unless toggle ON +
  admin; the NodeView executes nothing and shows a 'disabled in this workspace'
  placeholder when OFF; an admin Switch in Workspace Settings -> General with a
  description of the behavior.
- docs/html-embed-admin.md documents the toggle + admin-only + fail-closed
  coedit (a non-admin save strips an admin's embed) + execution semantics.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/database/repos/workspace/workspace.repo.ts

@@ -265,6 +265,32 @@ export class WorkspaceRepo {
       .executeTakeFirst();
   }
 
+  /**
+   * Set a single scalar key at the TOP LEVEL of `settings` (e.g.
+   * `settings.htmlEmbed`). Mirrors `updateAiSettings`/`updateSharingSettings`
+   * but without a nested namespace object. `prefKey` comes from a fixed
+   * allowlist at the call site (inlined via `sql.raw`, never user input); the
+   * value is inlined via `sql.lit`.
+   */
+  async updateSetting(
+    workspaceId: string,
+    prefKey: string,
+    prefValue: string | boolean,
+    trx?: KyselyTransaction,
+  ) {
+    const db = dbOrTx(this.db, trx);
+    return db
+      .updateTable('workspaces')
+      .set({
+        settings: sql`COALESCE(settings, '{}'::jsonb)
+                || jsonb_build_object('${sql.raw(prefKey)}', ${sql.lit(prefValue)})`,
+        updatedAt: new Date(),
+      })
+      .where('id', '=', workspaceId)
+      .returning(this.baseFields)
+      .executeTakeFirst();
+  }
+
   async updateSharingSettings(
     workspaceId: string,
     prefKey: string,