perf(indexes): add critical indexes for Test and AuditLog models (P0)

- Declare __table_args__ on Test with 5 indexes: technique_id, state, created_at, (technique_id,state), (state,created_at)

- Declare __table_args__ on AuditLog with 3 indexes: (entity_type,entity_id), timestamp, (entity_type,entity_id,action)

- Alembic b024: create only the 2 new indexes (ix_tests_created_at, ix_tests_state_created_at); existing indexes from b005/b018/b019 are preserved

- Model index names aligned with existing migration names to prevent duplicates

backend/alembic/versions/b024_add_critical_test_audit_indexes.py

@@ -0,0 +1,37 @@
+"""add_critical_test_audit_indexes
+
+Add missing critical indexes for tests and audit_logs tables to match
+model __table_args__ declarations.  Existing indexes (from b005, b018,
+b019) are left untouched; only the two genuinely new indexes are created.
+
+Revision ID: b024critidx
+Revises: b023mustchgpwd
+Create Date: 2026-02-18 12:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "b024critidx"
+down_revision: Union[str, None] = "b023mustchgpwd"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_index(
+        "ix_tests_created_at",
+        "tests",
+        ["created_at"],
+    )
+    op.create_index(
+        "ix_tests_state_created_at",
+        "tests",
+        ["state", "created_at"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_tests_state_created_at", table_name="tests")
+    op.drop_index("ix_tests_created_at", table_name="tests")