Address remaining CodeRabbit comments from PR #267 #351 (#451)

* Address remaining CodeRabbit comments from PR #267

This commit addresses the remaining unresolved code review comments:

1. Fix down migration in drop_was_merged_from_transactions.rb
   - Add null: false, default: false constraints to match original column
   - Ensures proper rollback compatibility

2. Fix bare rescue in maps_helper.rb compute_duplicate_only_flag
   - Replace bare rescue with rescue StandardError => e
   - Add proper logging for debugging
   - Follows Ruby best practices by being explicit about exception handling

These changes improve code quality and follow Rails/Ruby best practices.

* Refactor `SimplefinItemsController` and add tests for balances sync and account relinking behavior

- Replaced direct sync execution with `SyncJob` for asynchronous handling of balances sync.
- Updated account relinking logic to prevent disabling accounts with other active provider links.
- Removed unused `compute_relink_candidates` method.
- Added tests to verify `balances` action enqueues `SyncJob` and relinking respects account-provider relationships.

* Refactor balances sync to use runtime-only `balances_only` flag

- Replaced persistent `sync_stats` usage with runtime `balances_only?` predicate via `define_singleton_method`.
- Updated `SimplefinItemsController` `balances` action to pass `balances_only` flag to `SyncJob`.
- Enhanced `SyncJob` to attach transient `balances_only?` flag for execution.
- Adjusted `SimplefinItem::Syncer` logic to rely on the runtime `balances_only?` method.
- Updated controller tests to validate runtime flag usage in `SyncJob`.

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>

app/controllers/concerns/simplefin_items/maps_helper.rb

@@ -87,7 +87,8 @@ module SimplefinItems
           end
         end
         errs.present? && errs.all? { |m| m.to_s.downcase.include?("duplicate upstream account detected") }
-      rescue
+      rescue StandardError => e
+        Rails.logger.warn("SimpleFin maps: compute_duplicate_only_flag failed: #{e.class} - #{e.message}")
         false
       end
   end