Feat[BE]: add migration scripts to manage document columns in expenses table for Document service integration

internal/database/migrations/20251226031727_alter_table_expense_delete_document.down.sql

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+-- Rollback: restore document columns to expenses table
+ALTER TABLE expenses ADD COLUMN IF NOT EXISTS document_path JSON;
+ALTER TABLE expenses ADD COLUMN IF NOT EXISTS realization_document_path JSON;

internal/database/migrations/20251226031727_alter_table_expense_delete_document.up.sql

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+-- Delete document columns from expenses table since we now use Document service with polymorphic relations
+ALTER TABLE expenses DROP COLUMN IF EXISTS document_path;
+ALTER TABLE expenses DROP COLUMN IF EXISTS realization_document_path;