Set Project Budget

Body for PATCH /projects/{id}/budget — set the project budget cap.

ge=0: $0 is the explicit "pause everything" anchor (issue: user wants a one-knob off switch). At $0 every per-campaign allocation zeroes out, so the daily-limit gate in billing blocks all cold sends while campaigns keep their setup (status unchanged — paused, not deleted). The slider enforces the 0 ↔ positive-min stepping (no $1..min-1); the server stays permissive at >= 0.

The project's daily-budget anchor — the single number the dashboard bar shows and the slider sets. Per-campaign allocations are machine-managed and deliberately not surfaced; the old heavy per-campaign budget/spend/stats read was dropped (it only fed a now-removed page + a tiny "spent today" caption).