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/css/app.css

@@ -1414,6 +1414,45 @@ pre code {
   padding: 0.25rem 0.55rem;
 }
 
+.storage-edit-form {
+  position: absolute;
+  right: 1.5rem;
+  z-index: 30;
+  width: min(26rem, calc(100vw - 2rem));
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  align-items: end;
+  gap: 0.6rem;
+  padding: 0.85rem;
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  background: var(--card);
+  box-shadow: var(--shadow);
+}
+
+.storage-edit-form label {
+  display: grid;
+  gap: 0.25rem;
+}
+
+.storage-edit-form label span {
+  color: var(--muted-foreground);
+  font-size: 0.72rem;
+}
+
+.storage-edit-form .checkbox-field,
+.storage-edit-form button {
+  align-self: center;
+}
+
+@media (max-width: 720px) {
+  .storage-edit-form {
+    position: static;
+    grid-template-columns: 1fr;
+    width: 100%;
+  }
+}
+
 /* Badge variants */
 .badge-active {
   background: rgba(134, 239, 172, 0.12);

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;
   }