Perk override UI allows negative values (Ropecon: -2 meal tickets)

Reported by Ropecon 2026 in their post-con feedback: several program hosts ended up with exactly -2 meal tickets in their perks. Most likely user error, but the UI permits it.

The perk override number input (`src/components/involvement/PerksForm.tsx`, ``) has no `min` attribute and no clamping. A plausible mechanism for accidental changes: some browsers adjust a focused number input's value on mouse wheel scroll.

Fix:
* `min={0}` on the number input and clamp on change.
* Validate non-negative perk counts in the backend override mutation as well.