fix(server): clear auth cleanup intervals to prevent flaky test exits

[garvitkaushik-123]

Summary

Fixes the flaky require-admin-cross-tenant.test.ts that fails under full-suite parallelism but passes in isolation (#5393).

Root cause: The three setInterval timers started by startBackgroundCleanup in auth.ts (session cache, DB refresh, ban cache) are never cleared. When vitest exits while an interval callback is still in-flight, the worker can produce a corrupted response on the affected test.

Fix:

• startBackgroundCleanup now returns the timer handle instead of void
• All three timer handles are captured at module scope
• New stopAuthTimers() export calls clearInterval on all three
• Test adds afterAll(() => stopAuthTimers()) for clean teardown

Non-breaking — adds one optional export to auth.ts, no protocol surface change.

Test plan

• vitest run server/tests/unit/require-admin-cross-tenant.test.ts — 16/16 pass
• vitest run --no-file-parallelism server/tests/ — full sequential suite to confirm the race is gone
• tsc --noEmit — compiles clean

Closes #5393

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