feat(api): track previous in_progress_at during task status transitions

backend/migrations/versions/a2f6c9d4b7e8_add_previous_in_progress_at_to_tasks.py

@@ -0,0 +1,31 @@
+"""Track prior in_progress_at during status transitions.
+
+Revision ID: a2f6c9d4b7e8
+Revises: 4c1f5e2a7b9d
+Create Date: 2026-02-15 00:00:00.000000
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision = "a2f6c9d4b7e8"
+down_revision = "4c1f5e2a7b9d"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """Add previous_in_progress_at column to tasks."""
+    op.add_column(
+        "tasks",
+        sa.Column("previous_in_progress_at", sa.DateTime(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    """Drop previous_in_progress_at column from tasks."""
+    op.drop_column("tasks", "previous_in_progress_at")