Start and end balance anchors for historical account balances (#2455)

* Add kind field to valuation

* Fix schema conflict

* Add kind to valuation

* Scaffold opening balance manager

* Opening balance manager implementation

* Update account import to use opening balance manager + tests

* Update account to use opening balance manager

* Fix test assertions, usage of current balance manager

* Lint fixes

* Add Opening Balance manager, add tests to forward calculator

* Add credit card to "all cash" designation

* Simplify valuation model

* Add current balance manager with tests

* Add current balance logic to reverse calculator and plaid sync

* Tweaks to initial calc logic

* Ledger testing helper, tweak assertions for reverse calculator

* Update test assertions

* Extract balance transformer, simplify calculators

* Algo simplifications

* Final tweaks to calculators

* Cleanup

* Fix error, propagate sync errors up to parent

* Update migration script, valuation naming

app/models/balance/base_calculator.rb

@@ -0,0 +1,82 @@
+class Balance::BaseCalculator
+  attr_reader :account
+
+  def initialize(account)
+    @account = account
+  end
+
+  def calculate
+    raise NotImplementedError, "Subclasses must implement this method"
+  end
+
+  private
+    def sync_cache
+      @sync_cache ||= Balance::SyncCache.new(account)
+    end
+
+    def holdings_value_for_date(date)
+      holdings = sync_cache.get_holdings(date)
+      holdings.sum(&:amount)
+    end
+
+    def derive_cash_balance_on_date_from_total(total_balance:, date:)
+      if balance_type == :investment
+        total_balance - holdings_value_for_date(date)
+      elsif balance_type == :cash
+        total_balance
+      else
+        0
+      end
+    end
+
+    def derive_cash_balance(cash_balance, date)
+      entries = sync_cache.get_entries(date)
+
+      if balance_type == :non_cash
+        0
+      else
+        cash_balance + signed_entry_flows(entries)
+      end
+    end
+
+    def derive_non_cash_balance(non_cash_balance, date, direction: :forward)
+      entries = sync_cache.get_entries(date)
+      # Loans are a special case (loan payment reducing principal, which is non-cash)
+      if balance_type == :non_cash && account.accountable_type == "Loan"
+        non_cash_balance + signed_entry_flows(entries)
+      elsif balance_type == :investment
+        # For reverse calculations, we need the previous day's holdings
+        target_date = direction == :forward ? date : date.prev_day
+        holdings_value_for_date(target_date)
+      else
+        non_cash_balance
+      end
+    end
+
+    def signed_entry_flows(entries)
+      raise NotImplementedError, "Directional calculators must implement this method"
+    end
+
+    def balance_type
+      case account.accountable_type
+      when "Depository", "CreditCard"
+        :cash
+      when "Property", "Vehicle", "OtherAsset", "Loan", "OtherLiability"
+        :non_cash
+      when "Investment", "Crypto"
+        :investment
+      else
+        raise "Unknown account type: #{account.accountable_type}"
+      end
+    end
+
+    def build_balance(date:, cash_balance:, non_cash_balance:)
+      Balance.new(
+        account_id: account.id,
+        date: date,
+        balance: non_cash_balance + cash_balance,
+        cash_balance: cash_balance,
+        currency: account.currency
+      )
+    end
+end