Anti-raid and fail-safe toggles

[skylessdev]

Scope

• Enable Membership Screening; require captcha for non-verified.
• Add /maintenance on|off to disable new grants when RPC down.
• Rate-limit /verify per-user (e.g., 1/min).

Acceptance Criteria

• Raid test: 20 new accounts can’t spam chats.
• Maintenance mode blocks grants and replies with status hint.