User Story: Station Bot nomination digest delivery configuration

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User Story

As a guild owner or guild admin, I need to configure nomination digest delivery so that nomination summaries are posted to the right channel, mention the right role, and run on the right schedule for my guild.

Definition of Done

• Station exposes nominationDigestEnabled, nominationDigestChannelId, nominationDigestRoleId, and nominationDigestCronSchedule
• Channel/role validation and save/reload flows are covered by tests
• pnpm typecheck passes

Acceptance Criteria

• A guild admin can enable or disable nomination digest delivery from Station
• A guild admin can configure the digest channel, role mention, and schedule
• Invalid channel or role values are rejected with field-level feedback

Technical Elaboration

This ticket covers the guild-scoped nomination digest delivery settings from Station-Bot guild config. It is intentionally separate from delegated access, audit, anti-abuse, and worker controls.

Design Elaboration

This configuration should feel notification-centric. The admin should quickly understand where digests go, who gets notified, and when they run.

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