feat(server): impl doc gc (#15282)

#### PR Dependency Tree


* **PR #15282** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added automated document cleanup to reconcile missing workspace docs,
delete related stored data, and recover if the doc returns.
* Added effect-based follow-up reconciliation for search indexing,
Copilot embeddings, and comment attachment cleanup with explicit
acknowledgements.
* **Bug Fixes**
* Deleted-document references now persist as dangling references rather
than disappearing.
* Improved document deletion flow to enforce permissions and ensure
authorized deletions succeed.
* **Tests**
* Expanded coverage for cleanup recovery, indexing/embedding
reconciliation, permissions, and reference semantics.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/models/copilot-context.ts

@@ -337,10 +337,14 @@ export class CopilotContextModel extends BaseModel {
   }
 
   async deleteWorkspaceEmbedding(workspaceId: string, docId: string) {
+    await this.purgeWorkspaceEmbedding(workspaceId, docId);
+    await this.fulfillEmptyEmbedding(workspaceId, docId);
+  }
+
+  async purgeWorkspaceEmbedding(workspaceId: string, docId: string) {
     await this.db.aiWorkspaceEmbedding.deleteMany({
       where: { workspaceId, docId },
     });
-    await this.fulfillEmptyEmbedding(workspaceId, docId);
   }
 
   async matchWorkspaceEmbedding(