chore(backend): ignore runtime storage in git/docker, remove dead getSafeFilename

Follow-ups from the codex review of #834:

- .gitignore: backend/storage/ is runtime-generated (media, previews,
  thumbnails, business docs) and was only partially ignored — E2E runs
  left it dangling as untracked, which is how ~12 MB of artifacts nearly
  landed in a commit. Ignore the whole directory (nothing under it is
  tracked); replaces the narrower business-docs rule.
- backend/.dockerignore: the granular storage/* rules missed
  storage/previews, so locally generated previews were copied into
  production images. Exclude storage entirely — the Dockerfile creates
  the needed directories itself (RUN mkdir -p, Dockerfile:96).
- fileSecurityUtils.js: remove getSafeFilename — zero callers across the
  repo, and its private extension whitelist silently drifted from the
  real validation paths (see #834), which is exactly the trap dead
  security code sets.

backend/src/utils/fileSecurityUtils.js

@@ -213,25 +213,6 @@ async function validateFileContent(filePath, expectedMimeType) {
   }
 }
 
-/**
- * Get safe filename for storage
- * @param {string} originalFilename - Original filename
- * @returns {string} - Safe filename
- */
-function getSafeFilename(originalFilename) {
-  const timestamp = Date.now();
-  const randomString = Math.random().toString(36).substring(2, 15);
-  const ext = path.extname(originalFilename).toLowerCase();
-
-  // Validate extension - including both image and video extensions
-  const validExtensions = ['.jpg', '.jpeg', '.png', '.webp', '.gif', '.svg', '.ico', '.mp4', '.m4v', '.webm', '.mov', '.avi'];
-  if (!validExtensions.includes(ext)) {
-    throw new Error('Invalid file extension');
-  }
-
-  return `upload_${timestamp}_${randomString}${ext}`;
-}
-
 /**
  * Create a file upload validator middleware
  * @param {Object} options - Validation options
@@ -295,7 +276,6 @@ module.exports = {
   isPathSafe,
   validateFileType,
   validateFileContent,
-  getSafeFilename,
   createFileUploadValidator,
   ALLOWED_IMAGE_TYPES,
   ALLOWED_VIDEO_TYPES,