!!! Blocked By: #924, #925, #926 !!!

Description:
Create admin CRUD interface for PortfolioTransactions with dynamic enum select and currency formatting.

Tasks:

• Create controller: Admin::PortfolioTransactionsController
• Create views: show, new, edit (no index per routes config)
• Setup routes (except index)
• Implement transaction_type select that dynamically loads enum values
• Format amount_cents as currency ($ prefix, 2 decimals, 0.01 multiplier)
• Add read-only reason_humanized field
• Display BelongsTo portfolio association

Files to Create:

• app/controllers/admin/portfolio_transactions_controller.rb
• app/views/admin/portfolio_transactions/show.html.erb
• app/views/admin/portfolio_transactions/_form.html.erb

Reference:

• app/dashboards/portfolio_transaction_dashboard.rb
• app/models/portfolio_transaction.rb (for enum)