fix: linearize alembic migrations

backend/alembic/versions/0004_add_smtp_sender_email.py

@@ -0,0 +1,36 @@
+"""Add sender_email column to user_smtp_configs
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-05-03
+
+Adds a ``sender_email`` column to ``user_smtp_configs``.
+
+SMTP providers such as Postmark use an API token (UUID) as the SMTP username
+for authentication, but require a real email address as the ``From:`` header.
+This column stores the address that should appear as the sender; when blank,
+the existing ``username`` value is used as a fallback so existing rows remain
+fully functional.
+
+Using IF NOT EXISTS makes the migration idempotent against fresh installs
+where create_all() already created the column.
+"""
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "0004"
+down_revision = "0003"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute(
+        "ALTER TABLE user_smtp_configs "
+        "ADD COLUMN IF NOT EXISTS sender_email VARCHAR(255) NOT NULL DEFAULT ''"
+    )
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE user_smtp_configs DROP COLUMN IF EXISTS sender_email")