fix(#104): enforce storage quota on uploads & fix usage tracking

Backend:
- Add QuotaExceeded error kind mapped to HTTP 507 Insufficient Storage
- Add check_storage_quota() and get_user_storage_info() to StorageUsagePort
- Enforce quota in upload_file_with_cache, upload_file_with_thumbnails (AuthUser extractor)
- Enforce quota in chunked upload create_upload handler
- Add update_user_storage_usage_by_username() for username-based lookup
- Fix extract_username_from_path() to handle subfolders (take first segment only)
- Fix maybe_update_storage_usage() to use username-based lookup instead of passing
  username to get_user_by_id (which always failed silently)

Frontend:
- Parse and display quota error messages on upload failure (507 / QuotaExceeded)
- Stop remaining uploads when quota is exceeded
- Call refreshUserData() after uploads to update storage usage display
- Bump service worker cache to v9

src/domain/errors.rs

@@ -31,6 +31,8 @@ pub enum ErrorKind {
     UnsupportedOperation,
     /// Database error
     DatabaseError,
+    /// Storage quota exceeded
+    QuotaExceeded,
 }
 
 impl Display for ErrorKind {
@@ -45,6 +47,7 @@ impl Display for ErrorKind {
             ErrorKind::NotImplemented => write!(f, "Not Implemented"),
             ErrorKind::UnsupportedOperation => write!(f, "Unsupported Operation"),
             ErrorKind::DatabaseError => write!(f, "Database Error"),
+            ErrorKind::QuotaExceeded => write!(f, "Quota Exceeded"),
         }
     }
 }
@@ -162,6 +165,17 @@ impl DomainError {
         }
     }
 
+    /// Creates a storage quota exceeded error
+    pub fn quota_exceeded<S: Into<String>>(message: S) -> Self {
+        Self {
+            kind: ErrorKind::QuotaExceeded,
+            entity_type: "Storage",
+            entity_id: None,
+            message: message.into(),
+            source: None,
+        }
+    }
+
     /// Creates a validation error
     pub fn validation_error<S: Into<String>>(message: S) -> Self {
         Self {