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>

db/schema.rb

@@ -10,7 +10,35 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2025_09_28_181311) do
+ActiveRecord::Schema[8.0].define(version: 2025_09_29_222616) do
+  create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_uca1400_ai_ci", force: :cascade do |t|
+    t.string "name", null: false
+    t.string "record_type", null: false
+    t.bigint "record_id", null: false
+    t.bigint "blob_id", null: false
+    t.datetime "created_at", null: false
+    t.index ["blob_id"], name: "index_active_storage_attachments_on_blob_id"
+    t.index ["record_type", "record_id", "name", "blob_id"], name: "index_active_storage_attachments_uniqueness", unique: true
+  end
+
+  create_table "active_storage_blobs", charset: "utf8mb4", collation: "utf8mb4_uca1400_ai_ci", force: :cascade do |t|
+    t.string "key", null: false
+    t.string "filename", null: false
+    t.string "content_type"
+    t.text "metadata"
+    t.string "service_name", null: false
+    t.bigint "byte_size", null: false
+    t.string "checksum"
+    t.datetime "created_at", null: false
+    t.index ["key"], name: "index_active_storage_blobs_on_key", unique: true
+  end
+
+  create_table "active_storage_variant_records", charset: "utf8mb4", collation: "utf8mb4_uca1400_ai_ci", force: :cascade do |t|
+    t.bigint "blob_id", null: false
+    t.string "variation_digest", null: false
+    t.index ["blob_id", "variation_digest"], name: "index_active_storage_variant_records_uniqueness", unique: true
+  end
+
   create_table "events", charset: "utf8mb4", collation: "utf8mb4_uca1400_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "slug", null: false
@@ -130,6 +158,8 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_28_181311) do
     t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
   end
 
+  add_foreign_key "active_storage_attachments", "active_storage_blobs", column: "blob_id"
+  add_foreign_key "active_storage_variant_records", "active_storage_blobs", column: "blob_id"
   add_foreign_key "order_promotion_codes", "orders"
   add_foreign_key "order_promotion_codes", "promotion_codes"
   add_foreign_key "promotion_codes", "events"