feat(downloads): preserve original camera filenames on download (opt-in) (#493)

New Settings → General toggle `Use original filenames on download` (off by
default). When on, single-photo downloads, bulk/selection zips, and per-event
archive zips surface `photos.original_filename` instead of the sanitized
storage filename. Storage paths are unchanged.

- Content-Disposition uses RFC 5987 (`filename=` ASCII + `filename*=UTF-8''…`)
  so unicode camera filenames survive while header-injection bytes are stripped.
- Zip entries are deduplicated with a deterministic `_1` / `_2` suffix on
  collision (folder structure preserved in archive zips).
- Pre-generated download-all zips and the in-memory setting cache are
  invalidated when the toggle flips so the next download rebuilds with the
  new names.
- Falls back to the storage filename whenever `original_filename` is null
  (legacy uploads predating migration 062).

frontend/src/features/settings/hooks/useSettingsState.ts

@@ -20,6 +20,7 @@ export interface GeneralSettings {
   enable_registration: boolean;
   maintenance_mode: boolean;
   short_gallery_urls: boolean;
+  use_original_filenames_for_downloads: boolean;
   default_language: string;
   date_format: { format: string; locale: string };
 }
@@ -94,6 +95,7 @@ export function useSettingsState() {
     enable_registration: false,
     maintenance_mode: false,
     short_gallery_urls: false,
+    use_original_filenames_for_downloads: false,
     default_language: 'en',
     date_format: { format: 'dd/MM/yyyy', locale: 'en-GB' }
   });
@@ -179,6 +181,10 @@ export function useSettingsState() {
         enable_registration: toBoolean(settings.general_enable_registration, false),
         maintenance_mode: toBoolean(settings.general_maintenance_mode, false),
         short_gallery_urls: toBoolean(settings.general_short_gallery_urls, false),
+        use_original_filenames_for_downloads: toBoolean(
+          settings.general_use_original_filenames_for_downloads,
+          false
+        ),
         default_language: settings.general_default_language || 'en',
         date_format: settings.general_date_format
           ? (typeof settings.general_date_format === 'string'

frontend/src/features/settings/tabs/GeneralTab.tsx

@@ -229,6 +229,21 @@ export const GeneralTab: React.FC<GeneralTabProps> = ({
               {t('settings.general.enableShortGalleryUrlsHelp')}
             </p>
           </div>
+
+          <div>
+            <label className="flex items-center">
+              <input
+                type="checkbox"
+                checked={generalSettings.use_original_filenames_for_downloads}
+                onChange={(e) => setGeneralSettings(prev => ({ ...prev, use_original_filenames_for_downloads: e.target.checked }))}
+                className="w-4 h-4 text-primary-600 rounded focus:ring-primary-500"
+              />
+              <span className="ml-2 text-sm text-neutral-700 dark:text-neutral-300">{t('settings.general.useOriginalFilenames')}</span>
+            </label>
+            <p className="text-xs text-neutral-500 dark:text-neutral-400 ml-6 mt-1">
+              {t('settings.general.useOriginalFilenamesHelp')}
+            </p>
+          </div>
         </div>
       </Card>
 

frontend/src/i18n/locales/de.json

@@ -1038,6 +1038,8 @@
       "enableRegistration": "Selbstregistrierung für Admins erlauben",
       "enableShortGalleryUrls": "Kurze Galerie-Links verwenden",
       "enableShortGalleryUrlsHelp": "Entfernt den Veranstaltungs-Slug aus neuen Freigabelinks und lässt bestehende Links weiterhin funktionieren.",
+      "useOriginalFilenames": "Originale Dateinamen beim Download verwenden",
+      "useOriginalFilenamesHelp": "Wenn aktiviert, verwenden Einzel- und ZIP-Downloads den Original-Dateinamen der Kamera (z. B. DSC_1234.jpg) statt des umbenannten Namens. Der Speicher bleibt unverändert; Duplikate innerhalb einer Veranstaltung erhalten ein numerisches Suffix.",
       "maintenanceMode": "Wartungsmodus aktivieren",
       "language": "Sprache",
       "defaultLanguageHelp": "Sprache, die Gästen vor der Anmeldung angezeigt wird",

frontend/src/i18n/locales/en.json

@@ -677,6 +677,8 @@
       "enableRegistration": "Allow self-registration for admins",
       "enableShortGalleryUrls": "Use short gallery URLs",
       "enableShortGalleryUrlsHelp": "Removes the event slug from new share links while keeping existing links working.",
+      "useOriginalFilenames": "Use original filenames on download",
+      "useOriginalFilenamesHelp": "When on, single-photo and ZIP downloads use the original camera filename (e.g. DSC_1234.jpg) instead of the sanitized name. Storage is unchanged; duplicates in the same event get a numeric suffix.",
       "maintenanceMode": "Enable maintenance mode",
       "language": "Language",
       "defaultLanguageHelp": "Language shown to guests before login",