User Story: Station Bot nomination worker runtime controls

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User Story

As a Station super admin, I need to manage nomination worker runtime controls so that I can enable, disable, and tune nomination background processing from Station.

Definition of Done

• Station exposes NOMINATION_WORKER_ENABLED, concurrency, batch size, stale lock ms, max attempts, and poll ms controls
• Validation and save/reload flows are covered by tests
• Guild owners/admins cannot access these controls
• pnpm typecheck passes

Acceptance Criteria

• A station_super_admin can enable or disable the nomination worker from Station
• A station_super_admin can update worker tuning values with validation
• Guild-scoped admins cannot access these controls

Technical Elaboration

This ticket covers the nomination worker runtime settings only. It is separate from anti-abuse controls and from guild-scoped nomination settings.

Design Elaboration

This UI should emphasize operational tuning rather than policy. The admin should understand that these values affect background processing behavior, not guild-level business rules.

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