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/views/transactions/_header.html.erb

@@ -15,6 +15,12 @@
     <% if entry.transaction.transfer? %>
       <%= icon "arrow-left-right", class: "mt-1" %>
     <% end %>
+
+    <% if entry.linked? %>
+      <span title="Linked with Plaid">
+        <%= icon("refresh-ccw", size: "sm") %>
+      </span>
+    <% end %>
   </div>
 
   <span class="text-sm text-secondary">