fix(accounting): PR #622 concerns — flag-cache, customer master gate, VAT-unconfigured, helpers, page cap

1. requireFeatureFlag now caches each flag for 10s (the accounting area is 10+
   gated endpoints); PUT /admin/feature-flags invalidates the cache so toggles
   still take effect immediately.
2. Customer routes (/quotes, /invoices, /contracts + their PDFs) now gate via
   getEffectiveFeaturesForCustomer — the global MASTER flag AND the per-customer
   override — instead of the per-customer column alone, via a shared
   customerFeatureAllowed() helper. Admin disabling a feature globally is now
   honoured for customers too.
4. Tax-report VAT-payable: when accounting_vat_registered is UNSET, stop guessing
   from grandTotalVat>0 (a zero-output-VAT quarter silently flipped to "not
   registered" and hid the reclaim). Treat null as "not configured":
   vatPayableMinor=null + vatRegistrationConfigured=false; the UI renders "—" and
   a "configure VAT registration" warning. Tests updated.
5. Shared upsertAppSetting() in utils/appSettings — the two adminSettings upsert
   loops use it, so the app_settings created_at class can't be re-introduced.
6. PDF rasterise per-file bound: getRenderedPagePath refuses pages beyond
   MAX_RENDERABLE_PAGES (200); page_count is capped to match at ingest, so a
   hostile high-page PDF can't drive an unbounded pager.
7. (no code) original_filename is only rendered via auto-escaped JSX; the two
   dangerouslySetInnerHTML sites are admin-authored content — paranoia pass clean.

Concerns 3 (foreign-VAT reclaim-country) and 8 (imap_pass plaintext) are PR-reply
/ doc items, addressed in the PR response, not code.

frontend/src/services/taxReport.service.ts

@@ -84,7 +84,12 @@ export interface TaxReportSummary {
   costGrossMinor: number;
   resultNetMinor: number;
   resultGrossMinor: number;
-  vatPayableMinor: number;
+  /** Whether `accounting_vat_registered` is configured. When false, the
+   *  report refuses to guess and `vatPayableMinor` is null. */
+  vatRegistrationConfigured?: boolean;
+  /** output VAT − reclaimable input VAT; `null` when VAT registration is
+   *  unconfigured (the UI shows "—" + a warning instead of a guess). */
+  vatPayableMinor: number | null;
 }
 
 /** A single row of the unified ledger (#5). Outgoing invoices carry