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/student_loan_processor.rb

@@ -0,0 +1,50 @@
+class PlaidAccount::Liabilities::StudentLoanProcessor
+  def initialize(plaid_account)
+    @plaid_account = plaid_account
+  end
+
+  def process
+    return unless student_loan_data.present?
+
+    account.loan.update!(
+      rate_type: "fixed",
+      interest_rate: student_loan_data["interest_rate_percentage"],
+      initial_balance: student_loan_data["origination_principal_amount"],
+      term_months: term_months
+    )
+  end
+
+  private
+    attr_reader :plaid_account
+
+    def account
+      plaid_account.account
+    end
+
+    def term_months
+      return nil unless origination_date && expected_payoff_date
+
+      ((expected_payoff_date - origination_date).to_i / 30).to_i
+    end
+
+    def origination_date
+      parse_date(student_loan_data["origination_date"])
+    end
+
+    def expected_payoff_date
+      parse_date(student_loan_data["expected_payoff_date"])
+    end
+
+    def parse_date(value)
+      return value if value.is_a?(Date)
+      return nil unless value.present?
+
+      Date.parse(value.to_s)
+    rescue ArgumentError
+      nil
+    end
+
+    def student_loan_data
+      plaid_account.raw_liabilities_payload["student"]
+    end
+end