Pre-deploy security/correctness fixes (port log E17)

- Remove dead kitchen->KDS internal API (api/internal.py, verify_internal_secret)
  — KDS reads kitchen_db directly (E16), nothing ever called this endpoint
- Add expires_at to dispute_attachments; public attachment links now expire
  after 30 days instead of staying valid forever (A4)
- Add services/upload_validation.py: sniff real file content via python-magic
  instead of trusting the client-supplied Content-Type header, plus a 20MB
  cap. Applied across invoices/logbook/food_flags/credit_notes/disputes
  upload endpoints (A5) — disputes previously had no file-type check at all
- Fix nginx client_max_body_size drift (800m -> the plan's intended 20m)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Dockerfile

@@ -10,6 +10,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     unixodbc \
     unixodbc-dev \
     postgresql-client \
+    libmagic1 \
     && curl -fsSL https://packages.microsoft.com/keys/microsoft.asc \
        | gpg --dearmor -o /usr/share/keyrings/microsoft-prod.gpg \
     && curl -fsSL https://packages.microsoft.com/config/debian/12/prod.list \