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/gamification/services/gamification_service.dart

@@ -152,11 +152,28 @@ class GamificationService {
 
   // ---------------------------------------------------------------------------
   // Submit Contribution
-  // POST /v1/gamification/submit-event  body: event data
+  // 1. Upload each image to /v1/upload/file → get back { url, fileId, ... }
+  // 2. POST /v1/gamification/submit-event with `media` (uploaded objects)
   // ---------------------------------------------------------------------------
   Future<void> submitContribution(Map<String, dynamic> data) async {
     final email = await _getUserEmail();
-    final body = <String, dynamic>{'user_id': email, ...data};
+
+    // Upload images if present
+    final rawPaths = (data['images'] as List?)?.cast<String>() ?? [];
+    final List<Map<String, dynamic>> uploadedMedia = [];
+
+    for (final path in rawPaths) {
+      final result = await _api.uploadFile(ApiEndpoints.uploadFile, path);
+      uploadedMedia.add(result);
+    }
+
+    // Build submission body — use `media` (server canonical field)
+    final body = <String, dynamic>{
+      'user_id': email,
+      ...Map.from(data)..remove('images'),
+      if (uploadedMedia.isNotEmpty) 'media': uploadedMedia,
+    };
+
     await _api.post(
       ApiEndpoints.contributeSubmit,
       body: body,