chore(accounting): PR #622 nits — stray artifact, dedupe requireFlag, IMAP poll backoff

1. Remove the committed test artifact backend/storage/business-docs/quote/2026/
   Q-2026-0001.pdf and gitignore backend/storage/business-docs/ so generated CRM
   docs can't be committed again.
2. adminLedger + adminExpenses dropped their local requireFlag copies and now
   import the shared (now cached) requireFeatureFlag middleware.
4. roundTripTest polls IMAP with ×1.5 backoff (cap 8s) instead of a flat 3s, so a
   30s test takes ~5 SELECT/SEARCH locks not ~10 (some servers throttle).

Nit 3 (dashboard + events pages still on the gallery-theme vars, not dark-mode-
swapped) is left as a documented follow-up per the review.

.gitignore

@@ -129,3 +129,6 @@ docker-compose.dev.yml
 
 # New layout development files
 new-layouts/
+
+# Generated CRM/accounting documents (runtime) — never commit
+backend/storage/business-docs/