User Story: Station Bot verification throttle and session-ttl runtime controls

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User Story

As a Station super admin, I need to manage verification throttles and session TTL so that I can tune verification abuse controls and session behavior from Station.

Definition of Done

• Station exposes VERIFY_RATE_LIMIT_PER_MINUTE, VERIFY_RATE_LIMIT_PER_HOUR, and VERIFY_SESSION_TTL_MINUTES
• 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 update verification throttles and session TTL from Station
• Invalid values are rejected with field-level validation
• Guild-scoped admins cannot access these controls

Technical Elaboration

This ticket covers verification throttles and session TTL only. It is separate from top-level runtime feature flags.

Design Elaboration

This UI should read like abuse-prevention tuning. Rate limits and TTL should be grouped with unit labels that are unambiguous.

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