Issue #61 — Bulk market operations (admin)

[GoSTEAN]

Title: [Admin] Implement bulk market pause and cancel endpoint

Tags: backend, admin, markets, feature

Description:
Admin can pause or cancel multiple markets in a single request, used during incidents or platform maintenance.

Acceptance Criteria:

• POST /admin/markets/bulk-pause (admin) — body: { marketIds: number[] } (max 50).
• POST /admin/markets/bulk-cancel (admin) — body: { marketIds: number[], reason: string }.
• Each market processed independently; failures logged but do not abort others.
• Returns { succeeded: number[], failed: { id, reason }[] }.
• Sends notification to all position holders of cancelled markets.

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

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