fix(server): transcript key & bump models (#15485)

#### PR Dependency Tree


* **PR #15485** 👈

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**
* Upgraded built-in AI actions and chat model selection to Gemini 3.7
Flash.
* Transcript attachments now retain storage keys and support secure
presigned URLs during processing.
* Completed transcript results preserve attachment metadata for reliable
access.

* **Bug Fixes**
* Improved transcript handling across uploads, retries, and native
processing.
  * Added safeguards for invalid or missing attachment URLs.

* **Maintenance**
* Added a migration to backfill storage keys for existing transcript
records without altering their URLs.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/storage.ts

@@ -59,14 +59,31 @@ export class CopilotStorage {
   ): Promise<StorageRuntimeGetObjectResult> {
     const name = `${userId}/${workspaceId}/${key}`;
     if (signedUrl) {
-      const presigned = await this.rt.presignGet('copilot', name);
-      if (presigned) {
-        return { redirectUrl: presigned.url };
-      }
+      const redirectUrl = await this.presignGet(userId, workspaceId, key);
+      if (redirectUrl) return { redirectUrl };
     }
     return this.rt.getObject('copilot', name);
   }
 
+  async presignGet(userId: string, workspaceId: string, key: string) {
+    return (
+      await this.rt.presignGet('copilot', `${userId}/${workspaceId}/${key}`)
+    )?.url;
+  }
+
+  keyFromUrl(userId: string, workspaceId: string, url: string) {
+    try {
+      const parsed = new URL(url);
+      const prefix = `/api/copilot/blob/${encodeURIComponent(userId)}/${encodeURIComponent(workspaceId)}/`;
+      if (parsed.pathname.startsWith(prefix)) {
+        return decodeURIComponent(parsed.pathname.slice(prefix.length));
+      }
+    } catch {
+      return;
+    }
+    return undefined;
+  }
+
   @CallMetric('ai', 'blob_delete')
   async delete(userId: string, workspaceId: string, key: string) {
     await this.rt.deleteObject('copilot', `${userId}/${workspaceId}/${key}`);