feat: Implement event image upload system for promoters

- Add Active Storage migrations for file attachments
- Update Event model to handle image uploads with validation
- Replace image URL fields with file upload in forms
- Add client-side image preview with validation
- Update all views to display uploaded images properly
- Fix JSON serialization to prevent stack overflow in API
- Add custom image validation methods for format and size
- Include image processing variants for different display sizes
- Fix promotion code test infrastructure and Stripe configuration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/javascript/controllers/event_form_controller.js

@@ -664,4 +664,37 @@ export default class extends Controller {
     this.hideMessage("geocoding-success")
     this.hideMessage("geocoding-progress")
   }
+
+  // Preview selected image
+  previewImage(event) {
+    const file = event.target.files[0]
+    if (!file) return
+
+    // Validate file type
+    if (!file.type.startsWith('image/')) {
+      alert('Veuillez sélectionner une image valide.')
+      event.target.value = ''
+      return
+    }
+
+    // Validate file size (5MB)
+    if (file.size > 5 * 1024 * 1024) {
+      alert('L\'image ne doit pas dépasser 5MB.')
+      event.target.value = ''
+      return
+    }
+
+    // Show preview
+    const reader = new FileReader()
+    reader.onload = (e) => {
+      const previewContainer = document.getElementById('image-preview')
+      const previewImg = document.getElementById('preview-img')
+
+      if (previewContainer && previewImg) {
+        previewImg.src = e.target.result
+        previewContainer.classList.remove('hidden')
+      }
+    }
+    reader.readAsDataURL(file)
+  }
 }