Performance improvements in balance sync cache (#1581)

* Performance improvements in balance sync cache

Balance::SyncCache#converted_holdings called account.holdings.map { |h| h.dup }
which duplicated every holding record into a new ActiveRecord object, converted
its currency, and stored the full object in a holdings_by_date array hash.
For an investment account with years of history this allocates 100,000+
AR objects on every sync - one per holding row - creating proportional GC
pressure that scaled with account age.

The only consumer of get_holdings(date) was BaseCalculator#holdings_value_for_date,
which immediately discarded the objects after calling .sum(&:amount). The
individual holding objects were never accessed for any other attribute.

Replace the dup-and-group approach with a single aggregation pass that stores
only the per-date sum:

  holdings_value_by_date: account.holdings.each_with_object(Hash.new(0)) do |h, totals|
    converted = Money.new(h.amount, h.currency).exchange_to(account.currency, date: h.date).amount
    totals[h.date] += converted
  end

Interface change: get_holdings(date) -> get_holdings_value(date) returns a
Numeric directly rather than an Array. BaseCalculator#holdings_value_for_date
is updated accordingly, and its own per-date memoization layer is removed
since holdings_value_by_date is already fully memoized at the SyncCache level.

* fall back to 1:1 rate in SyncCache when holding exchange rate is missing; update tests to use investment class

test/models/balance/sync_cache_test.rb

@@ -5,7 +5,7 @@ class Balance::SyncCacheTest < ActiveSupport::TestCase
     @family = families(:dylan_family)
     @account = @family.accounts.create!(
       name: "Test Account",
-      accountable: Depository.new,
+      accountable: Investment.new,
       currency: "USD",
       balance: 1000
     )
@@ -125,6 +125,50 @@ class Balance::SyncCacheTest < ActiveSupport::TestCase
     assert_in_delta 120.0, amounts[2], 0.01  # 100 EUR * 1.2
   end
 
+  # get_holdings_value
+
+  test "returns 0 for date with no holdings" do
+    cache = Balance::SyncCache.new(@account)
+    assert_equal 0, cache.get_holdings_value(Date.current)
+  end
+
+  test "sums holdings value for a single date" do
+    security = Security.create!(ticker: "TST", name: "Test")
+
+    @account.holdings.create!(security: security, date: Date.current, qty: 10, price: 100, amount: 1000, currency: "USD")
+    @account.holdings.create!(security: security, date: 1.day.ago.to_date, qty: 10, price: 90, amount: 900, currency: "USD")
+
+    cache = Balance::SyncCache.new(@account)
+    assert_equal 1000, cache.get_holdings_value(Date.current)
+    assert_equal 900, cache.get_holdings_value(1.day.ago.to_date)
+  end
+
+  test "sums multiple holdings on the same date" do
+    s1 = Security.create!(ticker: "S1", name: "Security 1")
+    s2 = Security.create!(ticker: "S2", name: "Security 2")
+
+    @account.holdings.create!(security: s1, date: Date.current, qty: 10, price: 100, amount: 1000, currency: "USD")
+    @account.holdings.create!(security: s2, date: Date.current, qty: 5, price: 200, amount: 1000, currency: "USD")
+
+    assert_equal 2000, Balance::SyncCache.new(@account).get_holdings_value(Date.current)
+  end
+
+  test "converts foreign currency holdings to account currency" do
+    ExchangeRate.create!(from_currency: "EUR", to_currency: "USD", date: Date.current, rate: 1.5)
+
+    security = Security.create!(ticker: "TST", name: "Test")
+    @account.holdings.create!(security: security, date: Date.current, qty: 1, price: 100, amount: 100, currency: "EUR")
+
+    assert_equal 150.0, Balance::SyncCache.new(@account).get_holdings_value(Date.current)
+  end
+
+  test "falls back to 1:1 conversion rate when exchange rate is missing for a foreign currency holding" do
+    security = Security.create!(ticker: "TST", name: "Test")
+    @account.holdings.create!(security: security, date: Date.current, qty: 1, price: 100, amount: 100, currency: "EUR")
+
+    assert_equal 100, Balance::SyncCache.new(@account).get_holdings_value(Date.current)
+  end
+
   test "prioritizes custom rate over fetched rate" do
     # Create fetched rate
     ExchangeRate.create!(