fix(server): embedding chunks primary key (#12416)

fix AI-131

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

- **Refactor**
  - Updated database schema to consolidate unique constraints into composite primary keys for embedding-related data, improving consistency.
  - Changed the relation in the Snapshot model to allow multiple embeddings.
  - Improved filtering logic for documents and snapshots based on embedding existence.
  - Reformatted SQL queries and schema attributes for improved readability; no changes to functionality.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/context/resolver.ts

@@ -371,7 +371,7 @@ export class CopilotContextRootResolver {
     if (this.context.canEmbedding) {
       const total = await this.db.snapshot.count({ where: { workspaceId } });
       const embedded = await this.db.snapshot.count({
-        where: { workspaceId, embedding: { isNot: null } },
+        where: { workspaceId, embedding: { some: {} } },
       });
       return { total, embedded };
     }