Fix foreign currency accounts using wrong exchange rate in balance sheet totals (#1010)

Balance sheet totals and accountable type summaries used a SQL JOIN on
exchange_rates matching only today's date, which returned NULL (defaulting
to 1:1) when no rate existed for that exact date. This caused foreign
currency accounts to show incorrect totals.

Changes:
- Refactor BalanceSheet::AccountTotals to batch-fetch exchange rates via
  ExchangeRate.rates_for, with provider fallback, instead of a SQL join
- Refactor Accountable.balance_money to use the same batch approach
- Add ExchangeRate.rates_for helper for deduplicated rate lookups
- Fix net worth chart query to fall back to the nearest future rate when
  no historical rate exists for a given date
- Add composite index on accounts (family_id, status, accountable_type)
- Reuse nearest cached exchange rate within a 5-day lookback window
  before calling the provider, preventing redundant API calls on
  weekends and holidays when providers return prior-day rates

https://claude.ai/code/session_01GyssBJxQqdWnuYofQRjUu8

Co-authored-by: Claude <noreply@anthropic.com>

db/migrate/20260217120000_add_composite_index_on_accounts_family_status_type.rb

@@ -0,0 +1,9 @@
+class AddCompositeIndexOnAccountsFamilyStatusType < ActiveRecord::Migration[7.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index :accounts, [ :family_id, :status, :accountable_type ],
+      name: "index_accounts_on_family_id_status_accountable_type",
+      algorithm: :concurrently
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2026_02_11_120001) do
+ActiveRecord::Schema[7.2].define(version: 2026_02_17_120000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -56,6 +56,7 @@ ActiveRecord::Schema[7.2].define(version: 2026_02_11_120001) do
     t.index ["currency"], name: "index_accounts_on_currency"
     t.index ["family_id", "accountable_type"], name: "index_accounts_on_family_id_and_accountable_type"
     t.index ["family_id", "id"], name: "index_accounts_on_family_id_and_id"
+    t.index ["family_id", "status", "accountable_type"], name: "index_accounts_on_family_id_status_accountable_type"
     t.index ["family_id", "status"], name: "index_accounts_on_family_id_and_status"
     t.index ["family_id"], name: "index_accounts_on_family_id"
     t.index ["import_id"], name: "index_accounts_on_import_id"