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).

backend/__tests__/services/downloadFilenameService.test.js

@@ -0,0 +1,76 @@
+/**
+ * Pure-logic tests for the #493 download-filename helpers that don't depend
+ * on the DB (those are covered by the route integration suite).
+ */
+
+const {
+  pickRawDownloadName,
+  getZipEntryNames,
+} = require('../../src/services/downloadFilenameService');
+
+describe('pickRawDownloadName', () => {
+  it('returns the storage filename when the toggle is off', () => {
+    expect(
+      pickRawDownloadName({ id: 1, filename: 'slug_001.jpg', original_filename: 'DSC_1.jpg' }, false)
+    ).toBe('slug_001.jpg');
+  });
+
+  it('returns original_filename when the toggle is on', () => {
+    expect(
+      pickRawDownloadName({ id: 1, filename: 'slug_001.jpg', original_filename: 'DSC_1.jpg' }, true)
+    ).toBe('DSC_1.jpg');
+  });
+
+  it('falls back to storage filename when original_filename is missing', () => {
+    expect(
+      pickRawDownloadName({ id: 1, filename: 'slug_001.jpg', original_filename: null }, true)
+    ).toBe('slug_001.jpg');
+  });
+
+  it('produces a stable last-resort name when both are missing', () => {
+    expect(pickRawDownloadName({ id: 42 }, true)).toBe('photo-42.jpg');
+  });
+});
+
+describe('getZipEntryNames', () => {
+  it('uses original filenames with deterministic suffixes on collision', () => {
+    const photos = [
+      { id: 1, filename: 'slug_001.jpg', original_filename: 'DSC_1234.jpg' },
+      { id: 2, filename: 'slug_002.jpg', original_filename: 'DSC_1234.jpg' },
+      { id: 3, filename: 'slug_003.jpg', original_filename: 'DSC_1235.jpg' },
+    ];
+    expect(getZipEntryNames(photos, true)).toEqual([
+      'DSC_1234.jpg',
+      'DSC_1234_1.jpg',
+      'DSC_1235.jpg',
+    ]);
+  });
+
+  it('falls back to storage filename per-photo when original is missing', () => {
+    const photos = [
+      { id: 1, filename: 'slug_001.jpg', original_filename: 'DSC_1.jpg' },
+      { id: 2, filename: 'slug_002.jpg', original_filename: null },
+    ];
+    expect(getZipEntryNames(photos, true)).toEqual([
+      'DSC_1.jpg',
+      'slug_002.jpg',
+    ]);
+  });
+
+  it('returns storage filenames when the toggle is off, dedup still applies', () => {
+    const photos = [
+      { id: 1, filename: 'a.jpg', original_filename: 'DSC_1.jpg' },
+      { id: 2, filename: 'a.jpg', original_filename: 'DSC_2.jpg' },
+    ];
+    expect(getZipEntryNames(photos, false)).toEqual(['a.jpg', 'a_1.jpg']);
+  });
+
+  it('sanitizes path-traversal attempts that sneak into original_filename', () => {
+    const photos = [
+      { id: 1, filename: 'slug_001.jpg', original_filename: '../etc/passwd' },
+    ];
+    const [name] = getZipEntryNames(photos, true);
+    expect(name).not.toContain('..');
+    expect(name).not.toContain('/');
+  });
+});