🧑‍💻(backend) add email filter to target subset of users for merge cmd

Add an email substring filter to the user merge management command,
allowing selection of a subset of users concerned by the merge process.

This enables safer incremental execution of the command by testing it
on a controlled group of users before applying it globally.

The goal is to validate behavior and ensure the merge process does not
introduce unexpected side effects or inconsistencies at scale.

src/backend/core/management/commands/merge_duplicate_users.py

@@ -36,17 +36,28 @@ class Command(BaseCommand):
             action="store_true",
             help="Simulate the merge without writing any changes to the database.",
         )
+        parser.add_argument(
+            "--email-filter",
+            type=str,
+            default=None,
+            help="Only merge users whose email contains this string (e.g. '@example.com').",
+        )
 
     def handle(self, *args, **options):
         """Execute the management command."""
         dry_run = options["dry_run"]
+        email_filter = options["email_filter"]
 
         if dry_run:
             self.stdout.write("[DRY-RUN] No changes will be written.\n")
 
+        users_qs = User.objects.all()
+        if email_filter:
+            users_qs = users_qs.filter(email__icontains=email_filter)
+            self.stdout.write(f"[INFO] Filtering emails containing '{email_filter}'.\n")
+
         duplicate_emails = (
-            User.objects.all()
-            .exclude(email__isnull=True)
+            users_qs.exclude(email__isnull=True)
             .exclude(email="")
             .values("email")
             .annotate(cnt=Count("id"))

src/backend/core/tests/management/test_management_merge_duplicate_users.py

@@ -427,3 +427,34 @@ def test_merge_is_atomic_rolls_back_all_on_any_failure(mock_reassign_files):
     for f in files:
         f.refresh_from_db()
         assert f.creator == user1
+
+
+# ── Email filter ───────────────────────────────────────────────────────────────
+
+
+def test_merge_email_filter_only_merges_matching_emails():
+    """Command should only merge users whose email matches the filter."""
+    UserFactory(email="user1@example.com")
+    UserFactory(email="user1@example.com")
+    other1 = UserFactory(email="user1@other.com")
+    other2 = UserFactory(email="user1@other.com")
+    call_command("merge_duplicate_users", email_filter="@example.com")
+    assert User.objects.filter(email="user1@example.com").count() == 1
+    assert User.objects.filter(id=other1.id).exists()
+    assert User.objects.filter(id=other2.id).exists()
+
+
+def test_merge_email_filter_no_match_does_nothing():
+    """Command should do nothing when the email filter matches no users."""
+    UserFactory(email="user1@example.com")
+    UserFactory(email="user1@example.com")
+    call_command("merge_duplicate_users", email_filter="@nomatch.com")
+    assert User.objects.filter(email="user1@example.com").count() == 2
+
+
+def test_merge_email_filter_is_case_insensitive():
+    """Command should match emails case-insensitively when filtering."""
+    UserFactory(email="user1@Example.com")
+    UserFactory(email="user1@Example.com")
+    call_command("merge_duplicate_users", email_filter="@example.com")
+    assert User.objects.filter(email="user1@Example.com").count() == 1