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.

backend/src/services/emailIntakeService.js

@@ -202,6 +202,9 @@ async function roundTripTest({ timeoutMs = 30000, intervalMs = 3000 } = {}) {
   const client = makeImapClient(cfg);
   await connectWithTimeout(client);
   const started = Date.now();
+  // Backoff (PR #622 nit 4): some IMAP servers throttle frequent SELECT/SEARCH.
+  // Grow the gap ×1.5 (cap 8s) so a 30s test does ~5 polls, not ~10.
+  let delay = intervalMs;
   try {
     // eslint-disable-next-line no-constant-condition
     while (true) {
@@ -219,7 +222,8 @@ async function roundTripTest({ timeoutMs = 30000, intervalMs = 3000 } = {}) {
         return { ok: false, sent: true, reason: 'not_received', recipient };
       }
       // eslint-disable-next-line no-await-in-loop
-      await new Promise((r) => setTimeout(r, intervalMs));
+      await new Promise((r) => setTimeout(r, delay));
+      delay = Math.min(Math.round(delay * 1.5), 8000);
     }
   } finally {
     await client.logout().catch(() => {});