fix(security): sanitize chunked-upload filename (GHSA-pc72-jf53-w28j)

The chunked video upload stored req.body.filename unmodified and later built
the merged path as path.join(tempDir, uploadMeta.filename). path.join does not
neutralise '../', so a filename like '../../uploads/logos/evil.svg' escaped the
temp dir on merge and overwrote arbitrary files. Requires admin with
photos.upload.

Fix: path.basename() the client filename in initializeUpload() and reject
names that collapse to nothing. Adds a regression test.

backend/__tests__/services/chunkedUploadFilename.test.js

@@ -0,0 +1,52 @@
+const path = require('path');
+const os = require('os');
+const fs = require('fs').promises;
+
+// Point storage at a throwaway temp dir before requiring the service so the
+// module-level getStoragePath() picks it up if evaluated.
+process.env.STORAGE_PATH = path.join(os.tmpdir(), `picpeak-chunk-test-${process.pid}`);
+
+const chunkedUpload = require('../../src/services/chunkedUploadService');
+
+describe('chunkedUploadService.initializeUpload filename sanitisation (GHSA-pc72-jf53-w28j)', () => {
+  afterAll(async () => {
+    await fs.rm(process.env.STORAGE_PATH, { recursive: true, force: true }).catch(() => {});
+  });
+
+  it('strips directory-traversal components from the stored filename', async () => {
+    const { uploadId } = await chunkedUpload.initializeUpload({
+      filename: '../../uploads/logos/evil.svg',
+      fileSize: 10,
+      mimeType: 'video/mp4',
+      eventId: 1,
+      totalChunks: 1,
+    });
+    const meta = chunkedUpload.getUploadStatus(uploadId);
+    // basename('../../uploads/logos/evil.svg') === 'evil.svg' — the traversal
+    // is gone, so path.join(tempDir, filename) can no longer escape tempDir.
+    expect(meta.filename).toBe('evil.svg');
+  });
+
+  it('keeps a normal filename intact', async () => {
+    const { uploadId } = await chunkedUpload.initializeUpload({
+      filename: 'clip.mp4',
+      fileSize: 10,
+      mimeType: 'video/mp4',
+      eventId: 1,
+      totalChunks: 1,
+    });
+    expect(uploadId).toBeTruthy();
+  });
+
+  it('rejects a filename that collapses to nothing', async () => {
+    await expect(
+      chunkedUpload.initializeUpload({
+        filename: '../',
+        fileSize: 10,
+        mimeType: 'video/mp4',
+        eventId: 1,
+        totalChunks: 1,
+      })
+    ).rejects.toThrow(/Invalid filename/);
+  });
+});

backend/src/services/chunkedUploadService.js

@@ -30,6 +30,16 @@ async function initializeUpload(options) {
     totalChunks
   } = options;
 
+  // Strip any directory components from the client-supplied filename. It is
+  // later joined onto the temp merge dir (path.join(tempDir, filename)), and
+  // path.join does NOT neutralise `../` — a filename like `../../uploads/
+  // logos/evil.svg` would escape the temp dir and overwrite arbitrary files
+  // (GHSA-pc72-jf53-w28j). basename() collapses it to the leaf name only.
+  const safeFilename = path.basename(String(filename || ''));
+  if (!safeFilename || safeFilename === '.' || safeFilename === '..') {
+    throw new Error('Invalid filename');
+  }
+
   // Generate unique upload ID
   const uploadId = crypto.randomUUID();
 
@@ -43,7 +53,7 @@ async function initializeUpload(options) {
   // Store upload metadata
   const uploadMeta = {
     uploadId,
-    filename,
+    filename: safeFilename,
     fileSize,
     mimeType,
     eventId,
@@ -59,7 +69,7 @@ async function initializeUpload(options) {
 
   logger.info('Initialized chunked upload', {
     uploadId,
-    filename,
+    filename: safeFilename,
     fileSize,
     expectedChunks,
     eventId