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/i18n/locales/de.json

@@ -3974,7 +3974,8 @@
       "income": "Einnahmen",
       "costs": "Ausgaben",
       "result": "Ergebnis",
-      "vatPayable": "MWST-Zahllast (Umsatz- − Vorsteuer)"
+      "vatPayable": "MWST-Zahllast (Umsatz- − Vorsteuer)",
+      "vatUnconfigured": "Die MWST-Registrierung ist nicht konfiguriert, daher kann die MWST-Zahllast nicht berechnet werden. Lege sie unter Einstellungen → Buchhaltung fest."
     },
     "cost": {
       "source": "Art",

frontend/src/i18n/locales/en.json

@@ -3974,7 +3974,8 @@
       "income": "Income",
       "costs": "Costs",
       "result": "Result",
-      "vatPayable": "VAT payable (output − input)"
+      "vatPayable": "VAT payable (output − input)",
+      "vatUnconfigured": "VAT registration isn’t configured, so VAT payable can’t be computed. Set it under Settings → Accounting."
     },
     "cost": {
       "source": "Type",