feat(modules): settings_overrides and module_state migration

Add migration 016 with two tables for the module system:
- settings_overrides: dotted-path config overrides set via admin UI
- module_state: per-module enabled/disabled state

Update testutil.TruncateAll to include new tables.

Ref #94

internal/testutil/testutil.go

@@ -80,6 +80,7 @@ func TruncateAll(t *testing.T, pool *pgxpool.Pool) {
 
 	_, err := pool.Exec(context.Background(), `
 		TRUNCATE
+			settings_overrides, module_state,
 			job_log, jobs, job_definitions, runners,
 			dag_cross_edges, dag_edges, dag_nodes,
 			audit_log, sync_log, api_tokens, sessions, item_files,