Set the caller's spend caps

Applies the supplied caps, one entry per bucket. Per field, null clears the cap (unlimited), an omitted field leaves the stored value alone, and 0 is a real cap of zero that blocks all spend in that bucket. Only the caller's own caps are affected; an operator-set ceiling is untouched and continues to apply. Enforcement is charge-then-block: a call already in flight may push spend slightly past a cap, and subsequent calls are refused once cumulative spend reaches it. Caps are read per request, so a change takes effect immediately.