feat(ai): wire up workspace RAG bulk reindex + manual "Reindex now"

The WORKSPACE_CREATE_EMBEDDINGS / WORKSPACE_DELETE_EMBEDDINGS jobs were
enqueued (on AI Search enable/disable) but had no AI_QUEUE handler, so
existing pages were never indexed ("Indexed 0 of N pages") and disabling
never purged embeddings.

- EmbeddingProcessor: handle WORKSPACE_CREATE_EMBEDDINGS (bulk reindex all
  live pages) and WORKSPACE_DELETE_EMBEDDINGS (purge workspace embeddings)
- EmbeddingIndexerService: add reindexWorkspace() (skips when embeddings
  unconfigured; per-page error isolation) and removeWorkspace()
- PageRepo.getIdsByWorkspace(), PageEmbeddingRepo.deleteByWorkspace()
- AiSettingsService.reindex() + admin-only POST /workspace/ai-settings/reindex
- Frontend: "Reindex now" button, service call and mutation
- Stable per-workspace jobId with remove-before-add so a stale job can't
  block future reindexes; cancel the delayed purge on enable/reindex so it
  can't wipe freshly-built embeddings

apps/server/src/core/workspace/services/workspace.service.ts

@@ -513,9 +513,24 @@ export class WorkspaceService {
     });
 
     if (after.aiSearch === true) {
-      await this.aiQueue.add(QueueJob.WORKSPACE_CREATE_EMBEDDINGS, {
-        workspaceId,
-      });
+      // Cancel any pending delayed purge from a previous disable so it can't
+      // wipe the embeddings we are about to (re)build. The purge is a delayed
+      // job, so remove() simply deletes it (returning 0 if it is absent, without
+      // throwing). The .catch only guards against transport/Redis errors.
+      await this.aiQueue
+        .remove(`ai-search-disabled-${workspaceId}`)
+        .catch(() => undefined);
+      // Stable jobId de-duplicates with the manual "Reindex now" path and with
+      // repeated enable toggles (one full reindex at a time).
+      await this.aiQueue.add(
+        QueueJob.WORKSPACE_CREATE_EMBEDDINGS,
+        { workspaceId },
+        {
+          jobId: `ai-reindex-${workspaceId}`,
+          removeOnComplete: true,
+          removeOnFail: true,
+        },
+      );
     } else if (after.aiSearch === false) {
       const deleteJobId = `ai-search-disabled-${workspaceId}`;
       await this.aiQueue.add(