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/transactions/processor.rb

@@ -0,0 +1,60 @@
+class PlaidAccount::Transactions::Processor
+  def initialize(plaid_account)
+    @plaid_account = plaid_account
+  end
+
+  def process
+    # Each entry is processed inside a transaction, but to avoid locking up the DB when
+    # there are hundreds or thousands of transactions, we process them individually.
+    modified_transactions.each do |transaction|
+      PlaidEntry::Processor.new(
+        transaction,
+        plaid_account: plaid_account,
+        category_matcher: category_matcher
+      ).process
+    end
+
+    PlaidAccount.transaction do
+      removed_transactions.each do |transaction|
+        remove_plaid_transaction(transaction)
+      end
+    end
+  end
+
+  private
+    attr_reader :plaid_account
+
+    def category_matcher
+      @category_matcher ||= PlaidAccount::Transactions::CategoryMatcher.new(family_categories)
+    end
+
+    def family_categories
+      @family_categories ||= begin
+        if account.family.categories.none?
+          account.family.categories.bootstrap!
+        end
+
+        account.family.categories
+      end
+    end
+
+    def account
+      plaid_account.account
+    end
+
+    def remove_plaid_transaction(raw_transaction)
+      account.entries.find_by(plaid_id: raw_transaction["transaction_id"])&.destroy
+    end
+
+    # Since we find_or_create_by transactions, we don't need a distinction between added/modified
+    def modified_transactions
+      modified = plaid_account.raw_transactions_payload["modified"] || []
+      added = plaid_account.raw_transactions_payload["added"] || []
+
+      modified + added
+    end
+
+    def removed_transactions
+      plaid_account.raw_transactions_payload["removed"] || []
+    end
+end