feat(server): attachment embedding (#13348)

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

* **New Features**
* Added support for managing "blobs" in Copilot context, including
adding and removing blobs via new GraphQL mutations and UI fields.
* Introduced tracking and querying of blob embeddings within workspaces,
enabling search and similarity matching for blob content.
* Extended Copilot context and workspace APIs, schema, and UI to display
and manage blobs alongside existing documents and files.

* **Bug Fixes**
* Updated context and embedding status logic to handle blobs, ensuring
accurate status reporting and embedding management.

* **Tests**
* Added and updated test cases and snapshots to cover blob embedding
insertion, matching, and removal scenarios.

* **Documentation**
* Updated GraphQL schema and TypeScript types to reflect new
blob-related fields and mutations.

* **Chores**
* Refactored and cleaned up code to support new blob entity and
embedding logic, including renaming and updating internal methods and
types.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/migrations/20250728033348_workspace_blob_embedding/migration.sql

@@ -0,0 +1,37 @@
+-- CreateTable
+
+/*
+  Warnings:
+
+  - The primary key for the `ai_workspace_embeddings` table will be changed. If it partially fails, the table could be left without primary key constraint.
+  - The primary key for the `ai_workspace_file_embeddings` table will be changed. If it partially fails, the table could be left without primary key constraint.
+
+*/
+DO $$
+    BEGIN
+        IF EXISTS (SELECT 1 FROM pg_tables WHERE tablename = 'ai_workspace_embeddings') AND
+           EXISTS (SELECT 1 FROM pg_tables WHERE tablename = 'ai_workspace_file_embeddings') THEN
+            CREATE TABLE "ai_workspace_blob_embeddings" (
+                "workspace_id" VARCHAR NOT NULL,
+                "blob_id" VARCHAR NOT NULL,
+                "chunk" INTEGER NOT NULL,
+                "content" VARCHAR NOT NULL,
+                "embedding" vector(1024) NOT NULL,
+                "created_at" TIMESTAMPTZ(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+                CONSTRAINT "ai_workspace_blob_embeddings_pkey" PRIMARY KEY ("workspace_id","blob_id","chunk")
+            );
+
+            -- CreateIndex
+            CREATE INDEX "ai_workspace_blob_embeddings_idx" ON "ai_workspace_blob_embeddings"
+                USING hnsw (embedding vector_cosine_ops);
+
+            -- AddForeignKey
+            ALTER TABLE "ai_workspace_blob_embeddings"
+                ADD CONSTRAINT "ai_workspace_blob_embeddings_workspace_id_blob_id_fkey"
+                FOREIGN KEY ("workspace_id", "blob_id")
+                REFERENCES "blobs"("workspace_id", "key")
+                ON DELETE CASCADE ON UPDATE CASCADE;
+        END IF;
+    END
+$$;