Fix "invisible" merchants (#262)

- Fix display issues on the settings -> merchants page.
Currently it was showing only user created merchants, not provider created ones.
  - SimpleFin had its own MerchantDetector class that directly called ProviderMerchant.find_or_create_by!
  - This was inconsistent with Plaid and Lunchflow which use the centralized import_adapter.find_or_create_merchant method
  - Different unique keys: SimpleFin used source + name, others used source + provider_merchant_id

app/controllers/family_merchants_controller.rb

@@ -4,7 +4,8 @@ class FamilyMerchantsController < ApplicationController
   def index
     @breadcrumbs = [ [ "Home", root_path ], [ "Merchants", nil ] ]
 
-    @family_merchants = Current.family.merchants.alphabetically
+    # Show all merchants assigned to transactions (both FamilyMerchant and ProviderMerchant)
+    @family_merchants = Current.family.assigned_merchants.alphabetically
 
     render layout: "settings"
   end

app/models/simplefin_entry/processor.rb

@@ -1,3 +1,5 @@
+require "digest/md5"
+
 class SimplefinEntry::Processor
   # simplefin_transaction is the raw hash fetched from SimpleFin API and converted to JSONB
   def initialize(simplefin_transaction, simplefin_account:)
@@ -100,6 +102,22 @@ class SimplefinEntry::Processor
 
 
     def merchant
-      @merchant ||= SimplefinAccount::Transactions::MerchantDetector.new(data).detect_merchant
+      # Use SimpleFin's clean payee data for merchant detection
+      payee = data[:payee]&.strip
+      return nil unless payee.present?
+
+      @merchant ||= import_adapter.find_or_create_merchant(
+        provider_merchant_id: generate_merchant_id(payee),
+        name: payee,
+        source: "simplefin"
+      )
+    rescue ActiveRecord::RecordInvalid => e
+      Rails.logger.error "SimplefinEntry::Processor - Failed to create merchant '#{payee}': #{e.message}"
+      nil
+    end
+
+    def generate_merchant_id(merchant_name)
+      # Generate a consistent ID for merchants without explicit IDs
+      "simplefin_#{Digest::MD5.hexdigest(merchant_name.downcase)}"
     end
 end