Add S3/MinIO (RustFS) support and compose updates

Introduce S3-compatible storage support and update local/dev configuration.

- .claude: add "WebSearch" to local settings
- backend/.env.example: add USE_S3 flag and detailed S3/MinIO (RustFS) example env vars + comments to explain dev vs production usage
- backend/vontor_cz/settings.py: refactor to use USE_S3 boolean; set STATIC_ROOT; configure STORAGES for local filesystem vs S3 backends; add handling for S3 endpoint (MinIO/RustFS) vs real AWS S3 including path-style addressing, URL construction, and sensible AWS defaults
- docker-compose.yml: add rustfs to service dependencies, include rustfs in networks, change frontend port mapping to 8001:80, and add rustfs-data volume binding

These changes enable using a local S3-compatible backend (RustFS/MinIO) in development while keeping the option to swap to real AWS S3 for production.

.claude/settings.local.json

@@ -18,7 +18,8 @@
       "Bash(node -e \"const r = require\\('react-icons/si'\\); const celery = Object.keys\\(r\\).filter\\(k => k.toLowerCase\\(\\).includes\\('celery'\\) || k.toLowerCase\\(\\).includes\\('worker'\\) || k.toLowerCase\\(\\).includes\\('task'\\)\\).slice\\(0,10\\); console.log\\(celery\\);\")",
       "Bash(Get-ChildItem -Path \"c:\\\\Users\\\\bruno\\\\Documents\\\\GitHub\\\\vontor-cz\\\\backend\\\\\" -Directory | Select-Object -ExpandProperty Name)",
       "PowerShell(Get-Command python)",
-      "PowerShell(python -c \"import django; print\\(django.__version__\\)\")"
+      "PowerShell(python -c \"import django; print\\(django.__version__\\)\")",
+      "WebSearch"
     ]
   }
 }