feat(workflows): seed invoice-dunning ladder as an editable built-in flow

Boot self-heal seeds the corrected gate-in-loop dunning graph (wait→due,
grace wait, invoice_paid check, confirm-no-payment gate, bounded reminder
loop with re-check, final notice) keyed on builtin_key='invoice_dunning',
sized from the reminder_first/second_days settings. Seeded DISABLED and
is_builtin: live reminder behaviour is UNCHANGED (the hardcoded scheduler
ladder still runs) — enabling it pre-cutover would double-send, so the
engine cutover is a deliberate follow-up. Idempotent (preserves admin edits).
Built-ins refuse delete (enforced in the CRUD route). Test covers seed shape
+ idempotency.

backend/server.js

@@ -897,6 +897,15 @@ async function startServer() {
       logger.warn('restore-settings self-heal failed at boot:', err.message);
     }
 
+    // Seed built-in workflows (the editable invoice-dunning flow). Disabled by
+    // default — live reminder behaviour is unchanged. See _workflowSeedBoot.js.
+    try {
+      const { seedBuiltinWorkflowsAtBoot } = require('./src/services/_workflowSeedBoot');
+      await seedBuiltinWorkflowsAtBoot(db, logger);
+    } catch (err) {
+      logger.warn('built-in workflow seed failed at boot:', err.message);
+    }
+
     // Install-from-backup trigger. If `RESTORE_ON_INSTALL` (or
     // `.txt`) exists in the /backup mount AND the DB is empty, run
     // the restore HERE before any admin UI surfaces. Lets admins