Merchants improvements (#594)

* FIX logos

* Implement merchant mods

* FIX confirm issue

* FIX linter

* Add recently seen merchants to re-add if needed

* Update merge.html.erb

* FIX do security check

* Add error handling for update failures.

app/models/merchant/merger.rb

@@ -0,0 +1,54 @@
+class Merchant::Merger
+  class UnauthorizedMerchantError < StandardError; end
+
+  attr_reader :family, :target_merchant, :source_merchants, :merged_count
+
+  def initialize(family:, target_merchant:, source_merchants:)
+    @family = family
+    @target_merchant = target_merchant
+    @merged_count = 0
+
+    validate_merchant_belongs_to_family!(target_merchant, "Target merchant")
+
+    sources = Array(source_merchants)
+    sources.each { |m| validate_merchant_belongs_to_family!(m, "Source merchant '#{m.name}'") }
+
+    @source_merchants = sources.reject { |m| m.id == target_merchant.id }
+  end
+
+  private
+
+    def validate_merchant_belongs_to_family!(merchant, label)
+      return if family_merchant_ids.include?(merchant.id)
+
+      raise UnauthorizedMerchantError, "#{label} does not belong to this family"
+    end
+
+    def family_merchant_ids
+      @family_merchant_ids ||= begin
+        family_ids = family.merchants.pluck(:id)
+        assigned_ids = family.assigned_merchants.pluck(:id)
+        (family_ids + assigned_ids).uniq
+      end
+    end
+
+  public
+
+  def merge!
+    return false if source_merchants.empty?
+
+    Merchant.transaction do
+      source_merchants.each do |source|
+        # Reassign family's transactions to target
+        family.transactions.where(merchant_id: source.id).update_all(merchant_id: target_merchant.id)
+
+        # Delete FamilyMerchant, keep ProviderMerchant (it may be used by other families)
+        source.destroy! if source.is_a?(FamilyMerchant)
+
+        @merged_count += 1
+      end
+    end
+
+    true
+  end
+end