docs: correct SOLVER_SAMPLE_CAP subrequest-cost guidance
The README told readers to raise SOLVER_SAMPLE_CAP to ~200 on the paid
plan, directly contradicting the 50-subrequest budget it had just
described: at ~2 subrequests per sampled record, 200 records is ~400
subrequests. Replace the figure with the per-record cost so the cap is
sized against the actual budget, and mirror the note in config.ts.