feat(credits): one-time trial grant so the ledger runs in production

- Ledger accepts kind='grant' (a server-owned balance grant, not a payment).
- OPT_TRIAL_CREDITS=N grants N credits once per tenant (idempotent key
  trial:<tenant>) on balance/structure, so reserve -> settle is exercised
  before any customer purchase.
- sql/020 extends the kind constraint (re-runnable).
- Tests for the grant helper; full suite 1345 passed.

sql/020_credit_grants.sql

@@ -0,0 +1,16 @@
+-- Trial / grant credits: a server-owned, idempotent balance grant that is not
+-- a payment. Extends the existing kind constraint; safe to re-run.
+DO $$
+DECLARE cname text;
+BEGIN
+  SELECT conname INTO cname
+    FROM pg_constraint
+   WHERE conrelid = 'opt_credit_events'::regclass
+     AND contype = 'c'
+     AND pg_get_constraintdef(oid) LIKE '%kind%';
+  IF cname IS NOT NULL THEN
+    EXECUTE format('ALTER TABLE opt_credit_events DROP CONSTRAINT %I', cname);
+  END IF;
+  ALTER TABLE opt_credit_events ADD CONSTRAINT opt_credit_events_kind_check
+    CHECK (kind IN ('verified_payment', 'grant', 'reserve', 'settle', 'release'));
+END $$;