Add composite template index on notification history

This is a temporary change to allow us to delete the old emergency alerts data.
The notification_history table has this foreign key constraint, which
makes trying to delete the template history for emergency alerts too slow to be
feasible:
```
"notification_history_templates_history_fkey" FOREIGN KEY (template_id, template_version) REFERENCES templates_history(id, version)
```

This commit adds a composite index which matches the foreign key
constraint.

Author: klssmith

migrations/.current-alembic-head

@@ -1 +1 @@
-0507_n_history_service_id_index
+0508_n_history_templates_index

migrations/versions/0508_n_history_templates_index.py

@@ -0,0 +1,22 @@
+"""
+Create Date: 2025-07-07 11:03:04.356066
+"""
+
+from alembic import op
+
+revision = '0508_n_history_templates_index'
+down_revision = '0507_n_history_service_id_index'
+
+
+def upgrade():
+    with op.get_context().autocommit_block():
+        op.execute(
+            "CREATE INDEX CONCURRENTLY IF NOT EXISTS ix_notification_history_template_composite on notification_history (template_id, template_version)"
+        )
+
+
+def downgrade():
+    with op.get_context().autocommit_block():
+        op.execute(
+            "DROP INDEX CONCURRENTLY IF EXISTS ix_notification_history_template_composite"
+        )