Fix SimpleFin integration bugs and improve code quality

- Fix upsert method to handle string/symbol keys with indifferent access
- Add missing show route and view for SimpleFin items
- Fix test fixtures to use correct user references
- Update test data to match real-world JSON format (string keys, BigDecimal)
- Apply code formatting and linting fixes (rubocop, erb_lint)
- Ensure all SimpleFin tests pass (16/16 passing)

test/controllers/simplefin_items_controller_test.rb

@@ -2,7 +2,7 @@ require "test_helper"
 
 class SimplefinItemsControllerTest < ActionDispatch::IntegrationTest
   setup do
-    sign_in users(:dylan)
+    sign_in users(:family_admin)
     @family = families(:dylan_family)
     @simplefin_item = SimplefinItem.create!(
       family: @family,