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);