Slow mode is an admin setting: slowModeSeconds, 0-300, on /admin (after #211 makes main's tests green) #214

Replaces #193, part of #13. #193 was parked twice because every preview deploy failed: bun test fails on main itself (the first-visit gzip budget in src/server.test.ts, #211), not because of the change. This waits for #211, so the preview can deploy. This issue adds the setting only; #194 enforces it.

Done:

• [ ] Software/so101/src/admin.ts gains slowModeSeconds (integer 0-300, default 0). It is validated like the other settings, persisted, and logged when changed.
• [ ] /admin shows it as a number field that the owner can edit and moderators can only read.
• [ ] src/admin.test.ts covers the validation (-1, 301 and 1.5 are refused) and the persistence.
• [ ] The page's first-visit bytes do not grow (the field is on /admin only). forge.json is untouched. bun --cwd Software/so101 test passes in full, and the preview deploys.

Parked by the crew: the preview's deploy failed in the place it ran, not in the change, and it could not be run again (pull #223 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.