Clarify backend data pipeline naming concepts (importers, processors, materializers, calculators, and syncers) (#2255)

* Rename MarketDataSyncer to MarketDataImporter

* Materializers

* Importers

* More reference replacements

test/models/holding/materializer_test.rb

@@ -0,0 +1,29 @@
+require "test_helper"
+
+class Holding::MaterializerTest < ActiveSupport::TestCase
+  include EntriesTestHelper
+
+  setup do
+    @family = families(:empty)
+    @account = @family.accounts.create!(name: "Test", balance: 20000, cash_balance: 20000, currency: "USD", accountable: Investment.new)
+    @aapl = securities(:aapl)
+  end
+
+  test "syncs holdings" do
+    create_trade(@aapl, account: @account, qty: 1, price: 200, date: Date.current)
+
+    # Should have yesterday's and today's holdings
+    assert_difference "@account.holdings.count", 2 do
+      Holding::Materializer.new(@account, strategy: :forward).materialize_holdings
+    end
+  end
+
+  test "purges stale holdings for unlinked accounts" do
+    # Since the account has no entries, there should be no holdings
+    Holding.create!(account: @account, security: @aapl, qty: 1, price: 100, amount: 100, currency: "USD", date: Date.current)
+
+    assert_difference "Holding.count", -1 do
+      Holding::Materializer.new(@account, strategy: :forward).materialize_holdings
+    end
+  end
+end