fix(upload): restore configurable batch-size for reverse proxies (#509)

Regression of #208. PR #214 (commit 02a46e0, re-merged at 9b7495e)
shipped the configurable `general_max_upload_batch_size_mb` setting so
users behind Cloudflare Tunnel and other reverse proxies with
per-request size caps could lower the chunked-upload size below their
proxy's limit. Six days later the "Merge main into beta for
release/beta-to-main" commit (28793bb) resolved its conflict by
keeping main's older tree — which silently deleted the migration
(072), the setting input on Settings → General, the i18n strings, the
`useSettingsState` field, and the read in PhotoUpload.tsx, putting the
hardcoded 500MB chunk back. Galleries fronted by Cloudflare have
quietly been broken on batch uploads since then.

Re-applying exactly the same change set:

- `backend/migrations/core/072_add_max_upload_batch_size.js`
  recreated, with a comment pointing at the regression in case the
  same merge accident happens again.
- `frontend/src/components/admin/PhotoUpload.tsx` line 168 now reads
  the setting from query cache and falls back to 95MB (Cloudflare-safe
  headroom under 100MB).
- `useSettingsState.ts`, `GeneralTab.tsx`, `en.json`, `de.json` —
  added the field to the state type + defaults + load path + the
  Site-Configuration input.

Existing installs are safe either way:
- Ran original 072 then lost the file: migrations table still has the
  filename, so the runner skips re-applying. The setting row in
  `app_settings` is also untouched (the deletion was source-only, no
  down migration ran). Now the new code starts reading it again.
- Installed after the regression: migrations runner picks up the new
  072 normally and seeds the setting at 95.

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

@@ -16,6 +16,8 @@ export interface GeneralSettings {
   max_file_size_mb: number;
   max_files_per_upload: number;
   allowed_file_types: string;
+  // #509 — re-added after the main-into-beta merge dropped it.
+  max_upload_batch_size_mb: number;
   enable_analytics: boolean;
   enable_registration: boolean;
   maintenance_mode: boolean;
@@ -91,6 +93,7 @@ export function useSettingsState() {
     max_file_size_mb: 50,
     max_files_per_upload: 500,
     allowed_file_types: 'jpg,jpeg,png,gif,webp',
+    max_upload_batch_size_mb: 95,
     enable_analytics: true,
     enable_registration: false,
     maintenance_mode: false,
@@ -177,6 +180,7 @@ export function useSettingsState() {
           Math.max(1, toNumber(settings.general_max_files_per_upload, 500))
         ),
         allowed_file_types: settings.general_allowed_file_types || 'jpg,jpeg,png,gif,webp',
+        max_upload_batch_size_mb: toNumber(settings.general_max_upload_batch_size_mb, 95),
         enable_analytics: toBoolean(settings.general_enable_analytics, true),
         enable_registration: toBoolean(settings.general_enable_registration, false),
         maintenance_mode: toBoolean(settings.general_maintenance_mode, false),