Plaid sync domain improvements (#2267)

Breaks our Plaid sync process out into more manageable classes. Notably, this moves the sync process to a distinct, 2-step flow:

1. Import stage - we first make API calls and import Plaid data to "mirror" tables
2. Processing stage - read the raw data, apply business rules, build internal domain models and sync balances

This provides several benefits:

- Plaid syncs can now be "replayed" without fetching API data again
- Mirror tables provide better audit and debugging capabilities
- Eliminates the "all or nothing" sync behavior that is currently in place, which is brittle

app/models/plaid_account/liabilities/mortgage_processor.rb

@@ -0,0 +1,25 @@
+class PlaidAccount::Liabilities::MortgageProcessor
+  def initialize(plaid_account)
+    @plaid_account = plaid_account
+  end
+
+  def process
+    return unless mortgage_data.present?
+
+    account.loan.update!(
+      rate_type: mortgage_data.dig("interest_rate", "type"),
+      interest_rate: mortgage_data.dig("interest_rate", "percentage")
+    )
+  end
+
+  private
+    attr_reader :plaid_account
+
+    def account
+      plaid_account.account
+    end
+
+    def mortgage_data
+      plaid_account.raw_liabilities_payload["mortgage"]
+    end
+end