feat(storage): add S3 backend support and advanced upload limits

- Introduce S3-compatible storage backend support using minio-go.
- Add configuration options for local storage limits, box limits, and rate limiting.
- Implement storage backend selection (local vs S3) for anonymous and registered users.
- Add an `/admin/storage` management interface.
- Update documentation and environment examples with the new configuration variables.

backend/static/js/app.js

@@ -18,6 +18,7 @@
   const previewImages = document.querySelector("[data-preview-images]");
   const previewActions = document.querySelectorAll("[data-preview-action]");
   const fileContextMenu = document.querySelector("[data-file-context-menu]");
+  const storageProviderSelects = document.querySelectorAll("[data-storage-provider]");
   let ctrlCopyMode = false;
   let contextFile = null;
   const contextMenuCloseDistance = 80;
@@ -121,6 +122,30 @@
     });
   }
 
+  if (storageProviderSelects.length > 0) {
+    storageProviderSelects.forEach((select) => {
+      const formScope = select.closest("form");
+      const syncStorageProvider = () => {
+        if (!formScope) {
+          return;
+        }
+        const isContabo = select.value === "contabo";
+        const tls = formScope.querySelector('input[name="use_ssl"]');
+        const pathStyle = formScope.querySelector('input[name="path_style"]');
+        if (tls) {
+          tls.checked = isContabo || tls.checked;
+          tls.disabled = isContabo;
+        }
+        if (pathStyle) {
+          pathStyle.checked = isContabo || pathStyle.checked;
+          pathStyle.disabled = isContabo;
+        }
+      };
+      select.addEventListener("change", syncStorageProvider);
+      syncStorageProvider();
+    });
+  }
+
   if (!form || !dropZone || !fileInput) {
     return;
   }