Remove Flipper and replace with ENV-driven FeatureFlags (#957)

* Presence of valid DEFAULT_UI_LAYOUT is sufficient

* Linter

db/migrate/20260210120000_remove_flipper_tables.rb

@@ -0,0 +1,22 @@
+class RemoveFlipperTables < ActiveRecord::Migration[7.2]
+  def up
+    drop_table :flipper_gates, if_exists: true
+    drop_table :flipper_features, if_exists: true
+  end
+
+  def down
+    create_table :flipper_features do |t|
+      t.string :key, null: false
+      t.timestamps null: false
+    end
+    add_index :flipper_features, :key, unique: true
+
+    create_table :flipper_gates do |t|
+      t.string :feature_key, null: false
+      t.string :key, null: false
+      t.text :value
+      t.timestamps null: false
+    end
+    add_index :flipper_gates, [ :feature_key, :key, :value ], unique: true, length: { value: 255 }
+  end
+end