fix(gate-b): fresh-db install path - conditional guards on plugin-table migrations + dual-path convergence migrations

app/plugins/builtins/report_generator/migrations/0002_reports_folder_id.sql

@@ -0,0 +1,5 @@
+-- Dual-path convergence (Gate B): add the folder_id column that Alembic
+-- migration 0139 adds on the core path. Idempotent so both install paths
+-- converge to the identical schema.
+ALTER TABLE report_templates ADD COLUMN IF NOT EXISTS folder_id UUID;
+CREATE INDEX IF NOT EXISTS ix_report_templates_folder ON report_templates(folder_id);