newOS4 v4.6.18: nginx dav trailing-slash semantics

Discovered by dav-health-check.sh E2E (10/10 pass now):
- nginx dav module REQUIRES trailing slash on MKCOL (no-slash → 409)
  and on directory DELETE (no-slash → 409/404); existing dir + slash
  MKCOL → 405 (the real 'already exists' signal)
- _normalizeRemotePath now PRESERVES trailing slashes (collection ops)
- _ensureDir MKCOLs with trailing slash
- _deleteFile retries with slash on 409/404 (collection delete)
- health-check expectations corrected (405 = existing, slash required)

0 files changed, 0 insertions, 0 deletions