Add investment activity detection, labels, and exclusions

- Introduced `InvestmentActivityDetector` to mark internal investment activity as excluded from cashflow and assign appropriate labels.
- Added `exclude_from_cashflow` flag to `entries` and `investment_activity_label` to `transactions` with migrations.
- Implemented rake tasks to backfill and clear investment activity labels.
- Updated `PlaidAccount::Investments::TransactionsProcessor` to map Plaid transaction types to labels.
- Included comprehensive test coverage for new functionality.

db/migrate/20260110120000_add_investment_cashflow_support.rb

@@ -0,0 +1,13 @@
+class AddInvestmentCashflowSupport < ActiveRecord::Migration[7.2]
+  def change
+    # Flag for excluding from cashflow (user-controllable)
+    # Used for internal investment activity like fund swaps
+    add_column :entries, :exclude_from_cashflow, :boolean, default: false, null: false
+    add_index :entries, :exclude_from_cashflow
+
+    # Holdings snapshot for comparison (provider-agnostic)
+    # Used to detect internal investment activity by comparing holdings between syncs
+    add_column :accounts, :holdings_snapshot_data, :jsonb
+    add_column :accounts, :holdings_snapshot_at, :datetime
+  end
+end

db/migrate/20260110180000_add_investment_activity_label_to_transactions.rb

@@ -0,0 +1,8 @@
+class AddInvestmentActivityLabelToTransactions < ActiveRecord::Migration[7.2]
+  def change
+    # Label for investment activity type (Buy, Sell, Sweep In, Dividend, etc.)
+    # Provides human-readable context for why a transaction is excluded from cashflow
+    add_column :transactions, :investment_activity_label, :string
+    add_index :transactions, :investment_activity_label
+  end
+end