fix: calculateDayChange using real price history snapshots

[rookepoole]

Changes Included

1. backend/src/db/analyticsDb.ts:

   • Added dbGetLatestSnapshot, dbGetClosestSnapshot, and dbGetSnapshotAt query helpers.

2. backend/src/services/analyticsService.ts:

   • Replaced placeholder/hardcoded logic with real snapshot interpolation over defined time windows (e.g. 24h ± 2h).
   • Added calculateDayChange, calculateWeekChange, calculateMonthChange, calculateAllTimeChange, and calculatePeriodChange.
   • Extended calculatePerformanceMetrics to return null for dailyChange and weeklyChange when sufficient historical snapshot data is unavailable.
   • Updated captureSnapshot to persist captured snapshots to analyticsDb when SQL DB is active.

3. frontend/src/components/PerformanceChart.tsx & frontend/src/components/Dashboard.tsx:

   • Handled null change values gracefully in UI cards and performance chart metrics by displaying — instead of random or 0% values.

4. backend/src/test/analyticsService.test.ts:

   • Created unit tests verifying period change calculations and null returns when snapshots are outside the interpolation window.

Addresses #57

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