Every time in UTC, and every day as YYYY-MM-DD (closes #313)

AbuseIPDB's daily count keeps its day as a civil.Date, taken from the UTC
clock and written to reputation.json as "YYYY-MM-DD"; it was a time.Time
at midnight. Every time read from a state file is converted to UTC, so a
time an admin writes with an offset is written back in UTC.
TestMainRunsWAF runs sww waf with TZ 14 hours ahead of UTC and checks
that the process line, the request line and clients.json are in UTC.
SPEC.md states the rule near its start; AGENTS.md makes it an iron rule.

Model: opus-5-5

internal/reputation/abuseipdb_test.go

@@ -202,6 +202,43 @@ func TestDailyBudgetKeptAcrossARestartAndWholeAgainAsTheDayEnds(t *testing.T) {
 	})
 }
 
+func TestChecksAreCountedInUTCDaysWhateverTheClocksZone(t *testing.T) {
+	t.Parallel()
+
+	synctest.Test(t, func(t *testing.T) {
+		// A clock 14 hours ahead of UTC, whose day starts at 10:00 UTC.
+		ahead := time.FixedZone("UTC+14", 14*60*60)
+		p := abuseIPDBParams()
+		p.DailyBudget = 1
+		p.Now = func() time.Time { return time.Now().In(ahead) }
+		abuseIPDB := &abuseIPDBStandIn{scores: map[string]int64{suspect: 100}}
+		checker := newAbuseIPDB(abuseIPDB, p)
+
+		// At 09:00 UTC the one check is spent.
+		time.Sleep(9 * time.Hour)
+		hitFrom(t, checker, suspect, true)
+		synctest.Wait()
+		wantBudgetLeft(t, checker, 0)
+
+		// At 11:00 UTC the clock's day has changed, but the UTC day has not.
+		time.Sleep(2 * time.Hour)
+		wantBudgetLeft(t, checker, 0)
+
+		encoded, err := json.Marshal(checker.Snapshot())
+		if err != nil {
+			t.Fatalf("encode: %v", err)
+		}
+
+		if !strings.Contains(string(encoded), `"day":"2000-01-01"`) {
+			t.Errorf("reputation.json would hold %s, want the day 2000-01-01", encoded)
+		}
+
+		// At 00:00 UTC the budget is whole again.
+		time.Sleep(13 * time.Hour)
+		wantBudgetLeft(t, checker, 1)
+	})
+}
+
 func TestFailedCheckGivesNoScoreAndNoClientIsCheckedForAMinute(t *testing.T) {
 	t.Parallel()