User Story: Station Bot verification enablement and effective-state controls

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User Story

As a guild owner or guild admin, I need to control whether Station-Bot verification is enabled for my guild so that I can turn verification on or off from Station and understand when a global runtime gate overrides the guild setting.

Definition of Done

• Station exposes the guild-level verificationEnabled control in the verification admin screen
• The UI shows the effective state when a global runtime gate disables verification even if the guild toggle is on
• Save, reload, validation, and error states are covered by tests
• pnpm typecheck passes

Acceptance Criteria

• A guild admin can enable or disable verification for their guild from Station
• The saved value persists on reload
• If verification is globally disabled upstream, the UI shows that the guild setting is overridden instead of pretending the feature is writable and active

Technical Elaboration

This ticket covers the verificationEnabled guild configuration field from Station-Bot guild config. It should be implemented as its own saveable control and should participate in the normalized backend contract created in #232.

Design Elaboration

This control should be obvious and safe. The admin should immediately understand the difference between disabled for this guild and disabled globally by operator policy.

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