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

Every time the program stores, logs or compares is in UTC, whatever TZ says, and AbuseIPDB's day in `reputation.json` is a `YYYY-MM-DD` date. Times read from a state file are converted to UTC as they are read, so a file is written back with `Z` times only. A test runs `sww waf` with `TZ=Pacific/Kiritimati` and checks the process line, the request line and `clients.json`. `SPEC.md` states the rule and `AGENTS.md` makes it an iron rule.

- The rate limit's `day` and `day_bytes` buckets keep a start time: each is a 24-hour bucket begun at an instant, not a calendar day.
- New dependency `cloud.google.com/go`, of which only `civil` is built.
- No migration: a `reputation.json` holding the old day form fails to load, naming the file.

Model: opus-5-5

internal/state/state_test.go

@@ -14,6 +14,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"regexp"
 	"slices"
 	"strconv"
 	"strings"
@@ -21,6 +22,7 @@ import (
 	"testing/synctest"
 	"time"
 
+	"cloud.google.com/go/civil"
 	"sneak.berlin/go/smallwebwaf/internal/alerts"
 	"sneak.berlin/go/smallwebwaf/internal/anomaly"
 	"sneak.berlin/go/smallwebwaf/internal/bans"
@@ -258,7 +260,7 @@ const filledReputationJSON = `{
         }
     ],
     "abuseipdb": {
-        "day": "2026-10-06T00:00:00Z",
+        "day": "2026-10-06",
         "spent": 3,
         "scores": [
             {
@@ -1306,7 +1308,7 @@ func TestEditOfEachFileTakenIn(t *testing.T) {
 		`"tried": "2026-10-06T00:00:00Z", "fetched": "2026-10-06T00:00:00Z", `+
 		`"lines": ["198.51.100.7"]}], "verdicts": [{"zone": "`+dnsblZone+`", `+
 		`"client": "198.51.100.7", "listed": true, "fetched": "2026-10-06T00:00:00Z"}], `+
-		`"abuseipdb": {"day": "2026-10-06T00:00:00Z", "spent": 9, "scores": [`+
+		`"abuseipdb": {"day": "2026-10-06", "spent": 9, "scores": [`+
 		`{"client": "198.51.100.7/32", "score": 80, "fetched": "2026-10-06T00:00:00Z"}]}}`)
 	wantTakenIn(t, lines, dir, reputationJSON)
 
@@ -1321,7 +1323,7 @@ func TestEditOfEachFileTakenIn(t *testing.T) {
 	}})
 
 	checks := reputation.Checks{
-		Day: midnight(), Spent: 9,
+		Day: civil.DateOf(midnight()), Spent: 9,
 		Scores: []reputation.Score{{Client: client, Score: 80, Fetched: midnight()}},
 	}
 	if got := params.AbuseIPDB.Snapshot(); !reflect.DeepEqual(got, checks) {
@@ -1350,6 +1352,62 @@ func TestEditOfEachFileTakenIn(t *testing.T) {
 	}
 }
 
+func TestTimesEditedWithAnOffsetWrittenBackInUTC(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	params := newParams(dir)
+	fill(params)
+	// A lifted ban, so that bans.json holds every time a ban has.
+	params.Ledger.Lift(netip.MustParseAddr("192.0.2.1"), midnight())
+
+	files := load(t, params)
+
+	err := files.WriteAll()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	// An admin saves each file with every time in it two hours ahead of
+	// UTC, midnight as "2026-10-06T02:00:00+02:00".
+	stamp := regexp.MustCompile(`"\d{4}-\d\d-\d\dT[^"]*Z"`)
+	ahead := time.FixedZone("UTC+2", 2*60*60)
+	written := map[string]string{}
+
+	for _, name := range []string{
+		bansJSON, clientsJSON, lookupsJSON, reputationJSON, alertsJSON,
+	} {
+		written[name] = readFile(t, filepath.Join(dir, name))
+		edited := stamp.ReplaceAllStringFunc(written[name], func(quoted string) string {
+			at, err := time.Parse(time.RFC3339Nano, strings.Trim(quoted, `"`))
+			if err != nil {
+				t.Fatalf("%s holds %s: %v", name, quoted, err)
+			}
+
+			return `"` + at.In(ahead).Format(time.RFC3339Nano) + `"`
+		})
+
+		if !strings.Contains(edited, "+02:00") {
+			t.Fatalf("%s holds no time:\n%s", name, edited)
+		}
+
+		edit(t, dir, name, edited)
+	}
+
+	// Each write takes its edit in first, and writes the same times back in
+	// UTC.
+	err = files.WriteAll()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	for name, want := range written {
+		if got := readFile(t, filepath.Join(dir, name)); got != want {
+			t.Errorf("%s written back as\n%s\nwant\n%s", name, got, want)
+		}
+	}
+}
+
 func TestOwnWritesAreNotTakenIn(t *testing.T) {
 	t.Parallel()
 
@@ -1837,10 +1895,12 @@ func fill(params state.Params) {
 		},
 		{Zone: dnsblZone, Client: client.Addr(), Listed: true, Fetched: now.Add(-time.Hour)},
 	})
-	params.AbuseIPDB.Load(reputation.Checks{Day: now, Spent: 3, Scores: []reputation.Score{
-		{Client: netip.MustParsePrefix("2001:db8::/64"), Fetched: now.Add(-2 * time.Hour)},
-		{Client: client, Score: 100, Fetched: now.Add(-time.Hour)},
-	}})
+	params.AbuseIPDB.Load(reputation.Checks{
+		Day: civil.DateOf(now), Spent: 3, Scores: []reputation.Score{
+			{Client: netip.MustParsePrefix("2001:db8::/64"), Fetched: now.Add(-2 * time.Hour)},
+			{Client: client, Score: 100, Fetched: now.Add(-time.Hour)},
+		},
+	})
 
 	// An alert waiting, a repeat of it the cooldown holds back, another
 	// alert waiting, and one past the two an hour, for the hour's summary.