feat(db): add Phase 0 composite indexes migration [FASE-0.3]

Add idempotent Alembic revision b028 for campaign_tests (campaign_id, test_id) to support campaign-scoped queries.

backend/alembic/versions/b028_phase0_campaign_tests_composite_index.py

@@ -0,0 +1,35 @@
+"""phase0 SR-006 — campaign_tests composite index
+
+Most SR-006 indexes already ship in b005, b009, b018, b019, and b026.
+``tests`` has no ``campaign_id`` column (membership is ``campaign_tests``),
+so this revision adds a composite index to speed “tests in campaign” joins.
+
+Revision ID: b028phase0
+Revises: b027scorecfg
+Create Date: 2026-05-18 12:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "b028phase0"
+down_revision: Union[str, None] = "b027scorecfg"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_index(
+        "ix_campaign_tests_campaign_id_test_id",
+        "campaign_tests",
+        ["campaign_id", "test_id"],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(
+        "ix_campaign_tests_campaign_id_test_id",
+        table_name="campaign_tests",
+    )