feat: improve file upload authentication and error handling

- Add better error messages for authentication failures
- Display warning in ReportForm when user is not signed in
- Update documentation with authentication requirements
- Improve error handling in reportService.uploadPhoto()

This helps users understand they need to be signed in to upload photos.

server/FILE_UPLOAD_IMPLEMENTATION.md

@@ -100,14 +100,30 @@ Files are accessible at: `http://localhost:3005/uploads/{filename}`
 
 ### 1. Upload Photo
 
+**Important**: The user must be signed in with Clerk to upload photos. The frontend automatically includes the Clerk session token in the Authorization header.
+
+```typescript
+// Frontend usage (with Clerk authentication)
+const formData = new FormData();
+formData.append('photo', file);
+
+// The reportService automatically includes the auth token
+const { url, exifData } = await reportService.uploadPhoto(file);
+```
+
+**Manual API call** (for testing):
 ```typescript
 const formData = new FormData();
 formData.append('photo', file);
 
+// Get Clerk token (in a Clerk-authenticated component)
+const { getToken } = useAuth();
+const token = await getToken();
+
 const response = await fetch('/api/reports/upload', {
   method: 'POST',
   headers: {
-    'Authorization': `Bearer ${clerkToken}`
+    'Authorization': `Bearer ${token}`
   },
   body: formData
 });