Admin controls for the arm: pause the queue, end a turn, take over, turn length, speed cap, announcement banner #16

Goal (owner, 2026-10-20): complete, secure, public control of the real arm from the public forge URL, with a moderated community. Simple and amazing to use; every knob the owner might want is an admin setting.

Milestone 1. On /admin and as a compact admin bar on the main page (admins only): Pause/Resume the queue (maintenance - watchers see "paused"), End current turn, Take control now, Turn length (seconds), Idle timeout, Max speed (a cap on top of the safety limits), Max queue length, and an Announcement shown to everyone. All settings live, persisted, logged. Tests: pause stops new turns and shows the banner; end turn hands to the next; speed cap clamps commands; settings persist.