feat(enable-banking): safe pending transaction merge with sync re-import prevention (#1709)

* feat(enable-banking): safe pending transaction merge with sync re-import prevention

* preserve all merged pending IDs across syncs

* fix(enable-banking): harden merge locking, safe logging, and non-blocking index

* fix(enable-banking): use safe external ID in invalid currency log

* refactor(models): centralize pending transaction SQL logic

Move the SQL fragment used to identify pending transactions from the `Entry` model to a constant in the `Transaction` model. This improves maintainability and ensures that the logic for determining if a transaction is pending is defined in a single location.

* fix(enable-banking): drop dead manual_merge index, use lateral join for excluded IDs

* No net schema changes

---------

Co-authored-by: Juan José Mata <jjmata@jjmata.com>

app/controllers/transactions_controller.rb

@@ -180,7 +180,9 @@ class TransactionsController < ApplicationController
     end
 
     redirect_to transactions_path
-  rescue ActiveRecord::RecordNotDestroyed, ActiveRecord::RecordInvalid => e
+  rescue ActiveRecord::RecordNotFound, ActiveRecord::RecordInvalid,
+         ActiveRecord::RecordNotDestroyed, ActiveRecord::Deadlocked,
+         ActiveRecord::LockWaitTimeout => e
     Rails.logger.error("Failed to merge duplicate transaction #{params[:id]}: #{e.message}")
     flash[:alert] = t("transactions.merge_duplicate.failure")
     redirect_to transactions_path