Rename raw_investments_payload to raw_holdings_payload for Plaid accounts (#760)

* refactor: rename `raw_investments_payload` to `raw_holdings_payload`

- Update references and models to use consistent naming.
- Adjust migrations, tests, and encryption setup accordingly.

* fix: improve safety when accessing raw_holdings_payload keys

- Use `dig` with safe navigation to prevent potential nil errors.
- Add support for decryption from the old column name `raw_investments_payload`.
- Adjust related methods and calculations for consistency.

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Co-authored-by: luckyPipewrench <luckypipewrench@proton.me>

db/migrate/20260123214127_rename_raw_investments_payload_to_raw_holdings_payload.rb

@@ -0,0 +1,5 @@
+class RenameRawInvestmentsPayloadToRawHoldingsPayload < ActiveRecord::Migration[7.2]
+  def change
+    rename_column :plaid_accounts, :raw_investments_payload, :raw_holdings_payload
+  end
+end