Complete training workflow and acceptance hardening

backend/app/db/migrations/versions/f5a7d2c9b3e1_backend_session_leases.py

@@ -0,0 +1,35 @@
+"""add expiring backend ownership leases and fencing epochs
+
+Revision ID: f5a7d2c9b3e1
+Revises: e4b7c1d2a9f0
+Create Date: 2026-09-25
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "f5a7d2c9b3e1"
+down_revision = "e4b7c1d2a9f0"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "sessions",
+        sa.Column(
+            "backend_fencing_epoch",
+            sa.Integer(),
+            nullable=False,
+            server_default="0",
+        ),
+    )
+    op.add_column(
+        "sessions",
+        sa.Column("backend_lease_until", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("sessions", "backend_lease_until")
+    op.drop_column("sessions", "backend_fencing_epoch")