test(migrations): stop the pre-split backup test tracking the app version

`_backup_db` names its copy `homelab.db.back-<APP_VERSION>`, so the 3.4.1 bump
renamed the backup `init_db` takes from `back-3.3.2` to `back-3.4.1`. The test
hardcoded its post-split copies as `back-3.3.1` / `back-3.3.2`, which happened
to overwrite that backup while the version was 3.3.2; at 3.4.1 it survives,
still carrying the columns this very boot is about to drop, and wins the
newest-first scan in `_pre_split_backup`.

The fixture was also unlike a real install, where the current version's backup
is taken after the columns are already gone — they went in the 3.3.0 boot, not
this one. Copy a post-split database over it too, under whatever the version is,
and the test stops breaking on every bump.

`_pre_split_backup` is unchanged: preferring the newest backup that still holds
the columns is right, the drop boot's own backup included.

ha-relevant: no

backend/tests/migrations/test_legacy_node_columns.py

@@ -19,6 +19,7 @@ from sqlalchemy.ext.asyncio import create_async_engine
 
 import app.db.database as database
 import app.services.inventory_sync as inventory_sync
+from app.core.config import APP_VERSION
 
 # `create_all` under v3.2.0. NOT NULL is reproduced exactly where it was.
 _NODES_320 = (
@@ -412,8 +413,10 @@ async def test_the_newest_backup_that_still_has_the_columns_is_the_one_read(db_3
     shutil.copy2(db_path, pre_split)
 
     await database.init_db()  # drops the columns, and backs up under this version
-    # Post-split backups: newer, and unable to say who drew what.
-    for name in ("back-3.3.1", "back-3.3.2"):
+    # Post-split backups: newer, and unable to say who drew what. On a real
+    # install the current version's own backup is one of them — the columns went
+    # in an earlier boot, not this one — so overwrite the one `init_db` took.
+    for name in ("back-3.3.1", "back-3.3.2", f"back-{APP_VERSION}"):
         shutil.copy2(db_path, db_path.parent / f"{db_path.name}.{name}")
 
     assert database._pre_split_backup() == pre_split