fix(uploads): tighten guest max-file-size setting (codex review of #823)

Three follow-ups from the Codex review of #823:

1. PublicSettings TypeScript interface was missing general_max_file_size_mb,
   so UserPhotoUpload's access produced TS2339 under `tsc -b` (build:check). CI
   didn't catch it because the pipeline runs `build` (esbuild, no typecheck),
   but it's a real type gap — the #614 count field is declared, this one wasn't.
   Added the optional numeric field.

2. The general-settings update endpoint validated general_max_files_per_upload
   but not general_max_file_size_mb, so an out-of-range value (0, -1, huge)
   could persist. publicSettings then advertised the raw value while
   getMaxFileSizeMb() normalised it — the guest UI would reject files the
   backend accepts. Added the same validate-and-clamp block (1..MAX_ALLOWED_FILE_SIZE_MB).

3. The update route cleared the file-count cache but not the new file-size
   cache, so for up to 60s the public endpoint could advertise a new limit
   while multer still enforced the old one. Now clears both under the same
   uploadLimitTouched guard.

Follow-up on the merged #823 (main-only), so this targets main only.

backend/src/routes/adminSettings.js

@@ -25,7 +25,7 @@ const { resetSecurityConfigCache } = require('../utils/authSecurity');
 const { errorResponse } = require('../utils/routeHelpers');
 const logger = require('../utils/logger');
 const router = express.Router();
-const { clearMaxFilesPerUploadCache, MAX_ALLOWED_FILES_PER_UPLOAD } = require('../services/uploadSettings');
+const { clearMaxFilesPerUploadCache, MAX_ALLOWED_FILES_PER_UPLOAD, clearMaxFileSizeCache, MAX_ALLOWED_FILE_SIZE_MB } = require('../services/uploadSettings');
 const watermarkService = require('../services/watermarkService');
 const watermarkGeneratorService = require('../services/watermarkGeneratorService');
 
@@ -1095,6 +1095,24 @@ router.put('/general', adminAuth, requirePermission('settings.edit'), async (req
       settings.general_max_files_per_upload = normalizedValue;
     }
 
+    // Per-file size limit (MB). Validate/clamp on save, mirroring the count
+    // above, so an out-of-range value can't be persisted — otherwise the public
+    // endpoint would advertise the raw value while getMaxFileSizeMb() normalizes
+    // it, and the guest UI would reject files the backend actually accepts.
+    if (Object.prototype.hasOwnProperty.call(settings, 'general_max_file_size_mb')) {
+      uploadLimitTouched = true;
+      const rawValue = Number(settings.general_max_file_size_mb);
+      const normalizedValue = Number.isFinite(rawValue) ? Math.floor(rawValue) : NaN;
+
+      if (!Number.isInteger(normalizedValue) || normalizedValue < 1 || normalizedValue > MAX_ALLOWED_FILE_SIZE_MB) {
+        return res.status(400).json({
+          error: `general_max_file_size_mb must be an integer between 1 and ${MAX_ALLOWED_FILE_SIZE_MB}`
+        });
+      }
+
+      settings.general_max_file_size_mb = normalizedValue;
+    }
+
     if (publicSiteKeysTouched) {
       if (Object.prototype.hasOwnProperty.call(settings, 'general_public_site_custom_css')) {
         settings.general_public_site_custom_css = sanitizeCss(settings.general_public_site_custom_css || '');
@@ -1151,6 +1169,7 @@ router.put('/general', adminAuth, requirePermission('settings.edit'), async (req
     }
     if (uploadLimitTouched) {
       clearMaxFilesPerUploadCache();
+      clearMaxFileSizeCache();
     }
     if (Object.prototype.hasOwnProperty.call(settings, 'general_short_gallery_urls')) {
       clearShareLinkSettingsCache();

frontend/src/services/publicSettings.service.ts

@@ -82,6 +82,9 @@ export interface PublicSettings {
   // modal can render the real number in `upload.fileRequirements` and refuse
   // oversized batches client-side. Backend enforces the same value too.
   general_max_files_per_upload?: number;
+  // #613 follow-up — per-file size limit (MB), surfaced so the guest upload
+  // modal shows the real limit and guards client-side. Backend enforces it too.
+  general_max_file_size_mb?: number;
   // Event field requirements
   event_require_customer_name?: boolean;
   event_require_customer_email?: boolean;