feat: add last_login column to user table and update user login tracking

surfsense_backend/alembic/versions/103_add_last_login_to_user.py

@@ -0,0 +1,39 @@
+"""103_add_last_login_to_user
+
+Revision ID: 103
+Revises: 102
+Create Date: 2026-03-08
+
+Adds last_login timestamp column to the user table so we can track
+when each user last authenticated.  The column is nullable — existing
+rows will have NULL until the user's next login.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "103"
+down_revision: str | None = "102"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+    existing_columns = [col["name"] for col in sa.inspect(conn).get_columns("user")]
+
+    if "last_login" not in existing_columns:
+        op.add_column(
+            "user",
+            sa.Column("last_login", sa.TIMESTAMP(timezone=True), nullable=True),
+        )
+
+
+def downgrade() -> None:
+    op.drop_column("user", "last_login")