(core) Prune snapshots outside the window in product features

Summary:
- Add a method `getSnapshotWindow` to `IInventory` and `DocSnapshotInventory`. It returns a `SnapshotWindow`, which represents a duration of time for which we keep backups for a particular document.
- `DocSnapshotPruner` calls this method and passes the window to `shouldKeepSnapshots` to determine which document versions have fallen outside the window and should be pruned.
- The implementation passed to `DocSnapshotInventory` uses a new method `getDocProduct` in `HomeDBManager` which directly returns the `Product` associated with a document, given only the document ID. Other methods in `HomeDBManager` require passing more information, especially about a user, but `DocSnapshotPruner` only knows about document IDs.

Test Plan: Added a test for `getDocProduct` and a test for `DocSnapshotPruner` where `getSnapshotWindow` is specified.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3322

app/gen-server/lib/HomeDBManager.ts

@@ -2325,6 +2325,18 @@ export class HomeDBManager extends EventEmitter {
     });
   }
 
+  public async getDocProduct(docId: string): Promise<Product | undefined> {
+    return await this._connection.createQueryBuilder()
+      .select('product')
+      .from(Product, 'product')
+      .leftJoinAndSelect('product.accounts', 'account')
+      .leftJoinAndSelect('account.orgs', 'org')
+      .leftJoinAndSelect('org.workspaces', 'workspace')
+      .leftJoinAndSelect('workspace.docs', 'doc')
+      .where('doc.id = :docId', {docId})
+      .getOne();
+  }
+
   /**
    * Get the anonymous user, as a constructed object rather than a database lookup.
    */