Fix admin user null statistics

[sunnylqm]

What changed

• Treat Redis-backed user quota statistics as nullable in the admin API contract.
• Render unavailable quota statistics and app PV counts as - in the user detail drawer.

Why

When Redis statistics are unavailable, the server intentionally returns last7Days, today's quota values, and app check counts as null. The admin UI assumed those values were always present and crashed while reading last7Days.avg.

Impact

The /admin-users detail drawer now remains usable during Redis statistics degradation and clearly marks unavailable values.

Validation

• bun run typecheck
• bun run lint
• bun test (249 passed)
• bun run build

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