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.

.gitignore

@@ -130,5 +130,6 @@ docker-compose.dev.yml
 # New layout development files
 new-layouts/
 
-# Generated CRM/accounting documents (runtime) — never commit
-backend/storage/business-docs/
+# Backend runtime storage (generated media, previews, thumbnails,
+# CRM/accounting documents) — never commit
+backend/storage/