feat(contribute): upload event images to OneDrive before submission

- ApiClient.uploadFile() — multipart POST to /v1/upload/file (60s timeout)
- ApiEndpoints.uploadFile — points to Node.js upload endpoint
- GamificationService.submitContribution() now uploads each picked image
  to OneDrive via the server upload pipeline, then passes the returned
  { fileId, url, ... } objects as `media` in the submission body
  (replaces broken behaviour of sending local device paths as strings)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/features/events/models/event_models.dart

@@ -77,6 +77,10 @@ class EventModel {
   final String? contributorName;
   final String? contributorTier;
 
+  // Curation flags
+  final bool isFeatured;
+  final bool isTopEvent;
+
   EventModel({
     required this.id,
     required this.name,
@@ -105,6 +109,8 @@ class EventModel {
     this.contributorId,
     this.contributorName,
     this.contributorTier,
+    this.isFeatured = false,
+    this.isTopEvent = false,
   });
 
   /// Safely parse a double from backend (may arrive as String or num)
@@ -167,6 +173,8 @@ class EventModel {
       contributorId: j['contributor_id']?.toString(),
       contributorName: j['contributor_name'] as String?,
       contributorTier: j['contributor_tier'] as String?,
+      isFeatured: j['is_featured'] == true || j['is_featured']?.toString().toLowerCase() == 'true',
+      isTopEvent: j['is_top_event'] == true || j['is_top_event']?.toString().toLowerCase() == 'true',
     );
   }
 }