fix(core): mount backrest config directory instead of file

Backrest requires atomic file operations for config updates,
which fail with single-file bind mounts.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

core/compose.yaml

@@ -56,7 +56,7 @@ services:
     volumes:
       # Backrest application data
       - backrest_data:/data
-      - ./backrest/config.json:/config/config.json
+      - ./backrest:/config
       - backrest_cache:/cache
       - backrest_tmp:/tmp