Issue #55 — Admin dashboard stats

[GoSTEAN]

Title: [Admin] Implement admin dashboard statistics endpoint

Tags: backend, admin, feature

Description:
Aggregated platform-wide stats for the admin dashboard.

Acceptance Criteria:

• GET /admin/stats (admin only).
• Returns: { totalUsers, activeUsers24h, totalMarkets, openMarkets, totalVolume, totalFeeCollected, treasuryBalance, totalPredictions, totalDisputes, pendingDisputes }.
• Cached in Redis for 60 seconds.
• Integration test: create data → stats reflect accurate counts.

Files: src/routes/admin.routes.ts, src/controllers/admin.controller.ts, src/services/admin.service.ts

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