fix(billing): 未定价按天统计，让「已修好」和「还在漏」不再长得一样

计费页只有一个终身计数器 unpriced_reqs，实测 6378 —— 占 18981 请求的 34%。
但这个数字回答不了运维唯一会问的问题：「还在漏吗？」

原因是它把两件事混在一起：加规则之前的历史欠账，和加规则之后仍在发生的漏计。
修好一个源之后终身计数纹丝不动，于是仪表盘上「未定价 6378」和一个仍在
什么都没记的系统完全无法区分。

改动：
- 钩子按 UTC 天写入 unpriced_by_day{day:{reqs, models}}。终身计数保留
  （报表不该改写历史），天桶是新增的观察维度。
- 页面加「未定价（近 7 天）」一行。为 0 时额外显示「近 7 天无」——这时候
  零本身就是结论：加的规则确实生效了。
- 没有天桶数据时返回 null 而不是 0。旧的 state 文件说「未知」，
  不该假装有一份干净记录。

生产实测：全部视图 终身 6378 / 近 7 天 2；日视图隐藏终身计数（null）但保留
近 7 天 —— 周期视图原本把两者都置 null，因为那时根本没有天桶。

失败的请求同样计入（上游 502 的 gpt-5.5 也进了天桶）：一次失败的请求照样
消耗上游配额，只是没拿到 token，单看 token 维度会漏掉。

判据：两个不同日期的请求必须落到两个天桶且终身计数不重复累加；变异成
「总记今天」即变红（只判首日会漏掉这一点）。

Co-Authored-By: ModelRouter <noreply@modelrouter.dev>

internal/lua/billing_test.go

@@ -994,3 +994,49 @@ func TestBillingCacheHitClampedInStats(t *testing.T) {
 			tot["cache_fresh_tokens"])
 	}
 }
+
+// TestUnpricedIsTrackedPerDay pins the day-keyed unpriced tally.
+//
+// The lifetime counters cannot answer the question an operator actually has —
+// "is it STILL happening?" — because they keep the debt from before a rule was
+// added. After pricing a mispriced source the lifetime number never moves, so a
+// dashboard reading "unpriced: 6376" is indistinguishable from a system that is
+// still billing nothing. Day buckets are what make a pricing fix observable: the
+// offending day keeps its count and the next day starts at zero for it.
+func TestUnpricedIsTrackedPerDay(t *testing.T) {
+	ps, _ := billingVM(t)
+	// Nothing is priced in the default config, so every request is unpriced.
+	// Two requests on two different UTC days.
+	for _, ms := range []int64{1790000000000, 1790100000000} {
+		ps.Fire(StageRequestEnd, map[string]interface{}{
+			"model": "mystery-model", "source": "nowhere", "key": "***aaaaaa",
+			"ok": true, "prompt_tokens": 10, "completion_tokens": 5,
+			"time": ms,
+		})
+	}
+	st := stateOf(t, ps)
+	byDay, ok := st["unpriced_by_day"].(map[string]interface{})
+	if !ok {
+		t.Fatalf("state has no unpriced_by_day map: %T", st["unpriced_by_day"])
+	}
+	if len(byDay) != 2 {
+		t.Fatalf("expected 2 day buckets, got %d: %v", len(byDay), byDay)
+	}
+	var sum float64
+	for day, bucket := range byDay {
+		b := bucket.(map[string]interface{})
+		sum += b["reqs"].(float64)
+		models := b["models"].(map[string]interface{})
+		if models["mystery-model"].(float64) != 1 {
+			t.Errorf("%s: model tally = %v, want 1", day, models["mystery-model"])
+		}
+	}
+	if sum != 2 {
+		t.Errorf("day buckets sum to %v, want 2 (the lifetime counter must not be double-counted)", sum)
+	}
+	// The lifetime counters still exist and still agree — this is additive, not
+	// a replacement.
+	if st["unpriced_reqs"].(float64) != 2 {
+		t.Errorf("unpriced_reqs = %v, want 2", st["unpriced_reqs"])
+	}
+}