User Story: Station Bot temporary-member purge scheduling

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User Story

As a guild owner or guild admin, I need to manage temporary-member purge scheduling so that expiration windows and purge cadence match my guild’s onboarding process.

Definition of Done

• Station exposes purgeJobsEnabled, tempMemberHoursToExpire, and tempMemberPurgeCronSchedule
• Numeric/schedule validation and save/reload flows are covered by tests
• pnpm typecheck passes

Acceptance Criteria

• A guild admin can enable or disable purge jobs from Station
• A guild admin can update the temporary-member expiry window and purge schedule
• Invalid values are rejected with field-level validation

Technical Elaboration

This ticket covers the temp-member purge configuration cluster only. It is separate from event reminder delivery and reminder cleanup controls.

Design Elaboration

This UI should help admins understand both what expires and when the cleanup runs. It should feel operational and schedule-focused.

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Parent Epic: #231
Depends on: #232, #233, #242