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/loan.rb

@@ -1,6 +1,13 @@
 class Loan < ApplicationRecord
   include Accountable
 
+  SUBTYPES = {
+    "mortgage" => { short: "Mortgage", long: "Mortgage" },
+    "student" => { short: "Student", long: "Student Loan" },
+    "auto" => { short: "Auto", long: "Auto Loan" },
+    "other" => { short: "Other", long: "Other Loan" }
+  }.freeze
+
   def monthly_payment
     return nil if term_months.nil? || interest_rate.nil? || rate_type.nil? || rate_type != "fixed"
     return Money.new(0, account.currency) if account.loan.original_balance.amount.zero? || term_months.zero?