fix: linearize alembic migrations

backend/alembic/versions/0004_add_smtp_sender_email.py

@@ -1,7 +1,7 @@
 """Add sender_email column to user_smtp_configs
 
-Revision ID: 0003
-Revises: 0002
+Revision ID: 0004
+Revises: 0003
 Create Date: 2026-05-03
 
 Adds a ``sender_email`` column to ``user_smtp_configs``.
@@ -19,8 +19,8 @@ where create_all() already created the column.
 from alembic import op
 
 # revision identifiers, used by Alembic.
-revision = "0003"
-down_revision = "0002"
+revision = "0004"
+down_revision = "0003"
 branch_labels = None
 depends_on = None
 

backend/tests/unit/test_alembic_migrations.py

@@ -0,0 +1,14 @@
+from pathlib import Path
+
+from alembic.config import Config
+from alembic.script import ScriptDirectory
+
+
+def test_alembic_has_single_head() -> None:
+    backend_dir = Path(__file__).resolve().parents[2]
+    config = Config(str(backend_dir / "alembic.ini"))
+    config.set_main_option("script_location", str(backend_dir / "alembic"))
+
+    script = ScriptDirectory.from_config(config)
+
+    assert script.get_heads() == ["0004"]