Implement portfolio performance calculation (ROI, period returns, drawdown)

[snowrugar-beep]

Summary

Implements the Portfolio Performance Calculation work tracked by issue #326. Adds the missing Return-on-Investment (relative to invested cost basis), current allocation breakdown, live drawdown, and standard period-specific returns (YTD / 1Y / 3Y / 5Y) to the existing PerformanceAnalyticsService and surfaces each of them through dedicated controller endpoints.

Changes

• src/portfolio/services/performance-analytics.service.ts
  • New helpers: calculateROI, calculateCurrentDrawdown, calculatePeriodReturns, getAllocationBreakdown.
  • recordMetrics now enriches each persisted snapshot with YTD/1Y/3Y/5Y returns and the current drawdown.
  • getPerformanceSummary now also returns roi, currentDrawdown, allocationBreakdown, and periodReturns alongside the existing risk-adjusted metrics (additive, backward compatible).
  • calculateROI derives current value from quantity * currentPrice rather than the persisted value column, which can lag until updatePortfolioAllocation runs.
• src/portfolio/portfolio.controller.ts
  • Four new GET endpoints under each portfolio: metrics/roi, metrics/drawdown, metrics/periods, metrics/allocation.
• src/portfolio/services/performance-analytics.service.spec.ts
  • Extended unit coverage to include empty-history, null-allocation, zero-cost-basis, monotonically-rising/declining series, and peak-at-first/last edge cases.

Testing

• npx jest src/portfolio/services/performance-analytics.service.spec.ts — 57 / 57 passing.
• Coverage on the service file: 96.24% statements / 97.05% lines / 97.36% funcs / 87.05% branches (well above the >90% target).
• npx eslint on the three changed files — clean; only a pre-existing unused-import warning in portfolio.controller.ts, unrelated to this change.
• src/portfolio/ test suite re-verified end-to-end for regressions in the touched area.

Note: npx tsc --noEmit -p tsconfig.json reports a single type error in src/portfolio/services/trading-transaction.service.ts that also reproduces on the upstream main branch. That file is untouched by this PR and is left for a separate fix.

Closes #326