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/reputation/abuseipdb.go

@@ -14,6 +14,7 @@ import (
 	"sync"
 	"time"
 
+	"cloud.google.com/go/civil"
 	"github.com/hashicorp/golang-lru/v2/simplelru"
 	"sneak.berlin/go/smallwebwaf/internal/alerts"
 )
@@ -27,8 +28,6 @@ const (
 	AbuseIPDBSource = "abuseipdb"
 	// maxAnswerBytes is the most of an answer of AbuseIPDB that is read.
 	maxAnswerBytes = 64 << 10
-	// day is the length of the day the checks are counted in, in UTC.
-	day = 24 * time.Hour
 )
 
 var (
@@ -50,9 +49,9 @@ type Score struct {
 // AbuseIPDB: the day, in UTC, of the checks Spent counts, zero before the
 // first, and the scores still in use.
 type Checks struct {
-	Day    time.Time `json:"day,omitzero"`
-	Spent  int       `json:"spent"`
-	Scores []Score   `json:"scores"`
+	Day    civil.Date `json:"day,omitzero"`
+	Spent  int        `json:"spent"`
+	Scores []Score    `json:"scores"`
 }
 
 // AbuseIPDBParams are what NewAbuseIPDB needs.
@@ -93,7 +92,7 @@ type AbuseIPDB struct {
 	// checking are the clients whose check is under way.
 	checking map[netip.Prefix]bool
 	// day is the day, in UTC, of the checks spent counts.
-	day   time.Time
+	day   civil.Date
 	spent int
 	// checks and failures count the checks made and those that failed,
 	// and retryAt is when a client may be checked again after the last
@@ -143,7 +142,7 @@ func (a *AbuseIPDB) Hit(
 		return kept.Score, kept.Score >= a.params.MinScore
 	}
 
-	if today := now.Truncate(day); !a.day.Equal(today) {
+	if today := civil.DateOf(now.UTC()); a.day != today {
 		a.day, a.spent = today, 0
 	}
 
@@ -190,7 +189,7 @@ func (a *AbuseIPDB) BudgetLeft() int {
 	a.mu.Lock()
 	defer a.mu.Unlock()
 
-	if !a.day.Equal(a.params.Now().Truncate(day)) {
+	if a.day != civil.DateOf(a.params.Now().UTC()) {
 		return a.params.DailyBudget
 	}