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.

---------

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

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2026_01_23_000000) do
+ActiveRecord::Schema[7.2].define(version: 2026_01_23_214127) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -943,7 +943,7 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_23_000000) do
     t.datetime "updated_at", null: false
     t.jsonb "raw_payload", default: {}
     t.jsonb "raw_transactions_payload", default: {}
-    t.jsonb "raw_investments_payload", default: {}
+    t.jsonb "raw_holdings_payload", default: {}
     t.jsonb "raw_liabilities_payload", default: {}
     t.index ["plaid_id"], name: "index_plaid_accounts_on_plaid_id", unique: true
     t.index ["plaid_item_id"], name: "index_plaid_accounts_on_plaid_item_id"