实现月结账单生成

solve()

Monthly bills summary (2026-06):
  Total bills: 30
  Total impressions: 4138
  Total cost (before tax): 121639.35 CNY
  Total due (with tax): 128937.72 CNY
Top 3 advertisers by bill:
  ADV-003 REAL_ESTATE广告主03 due=22260.0 imps=256
  ADV-005 HEALTHCARE广告主05 due=20670.0 imps=266
  ADV-013 HEALTHCARE广告主13 due=19610.0 imps=261
Bill status: {'issued': np.int64(11), 'overdue': np.int64(11), 'paid': np.int64(8)}