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.

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;