fix(campaigns): preserve modification-request audit row after test deletion

Approving a remove_test modification request deletes the underlying Test
row, which was cascading through test_id's ON DELETE CASCADE and wiping
out the request row itself before it could be read back. Changed to
ON DELETE SET NULL so the audit record (justification, reviewer,
decision) survives. Adds regression coverage plus double-approve/reject
idempotency tests.

backend/alembic/versions/b056_mod_request_test_id_set_null.py

@@ -0,0 +1,50 @@
+"""Change campaign_modification_requests.test_id to nullable with ON DELETE SET NULL.
+
+Approving a "remove_test" modification request deletes the underlying Test
+row. With the original ON DELETE CASCADE, that delete cascaded and wiped
+out the modification-request row itself, destroying the audit record
+(justification, reviewer, decision) the request exists to preserve.
+
+Revision ID: b056
+Revises: b055
+Create Date: 2026-07-03
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "b056"
+down_revision = "b055"
+branch_labels = None
+depends_on = None
+
+_FK_NAME = "campaign_modification_requests_test_id_fkey"
+
+
+def upgrade() -> None:
+    op.alter_column(
+        "campaign_modification_requests", "test_id",
+        existing_type=sa.dialects.postgresql.UUID(as_uuid=True),
+        nullable=True,
+    )
+    op.drop_constraint(_FK_NAME, "campaign_modification_requests", type_="foreignkey")
+    op.create_foreign_key(
+        _FK_NAME,
+        "campaign_modification_requests", "tests",
+        ["test_id"], ["id"],
+        ondelete="SET NULL",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint(_FK_NAME, "campaign_modification_requests", type_="foreignkey")
+    op.create_foreign_key(
+        _FK_NAME,
+        "campaign_modification_requests", "tests",
+        ["test_id"], ["id"],
+        ondelete="CASCADE",
+    )
+    op.alter_column(
+        "campaign_modification_requests", "test_id",
+        existing_type=sa.dialects.postgresql.UUID(as_uuid=True),
+        nullable=False,
+    )