Bans an admin makes or lifts: the admin cause, a reason, lifted bans kept (closes #86)

A bans.json entry without a cause gets the cause admin, written back so.
Bans whose cause is admin are never dropped and do not count toward
SWWAF_MAX_BANS, so setting a ban's cause to admin keeps it. Bans
smallwebwaf makes get a reason: the limit broken or the rule matched. A
lifted ban refuses nothing, is kept, and makes no later ban longer.
smallwebwaf_bans_made_total counts admin bans an edit adds while running;
earlier_bans counts admin in place of without_cause.

Judgement call: lifted lifts at once, whatever time it gives.
Judgement call: a lifted ban still counts in earlier_bans.
Known gap: a ban dropped from behind an admin's ban on its netblock leaves that netblock's later earlier_bans.

Model: opus-5-5

internal/bans/bans.go

@@ -582,9 +582,10 @@ func (l *Ledger) attackExpiry(held []Ban, now time.Time) time.Time {
 // dropOne drops the earliest ban whose cause is not CauseAdmin of the
 // netblock that has gone longest without a request, of those that hold
 // such a ban, and the netblock with it if that was its only ban. It is
-// called with at least one such ban held.
+// called with at least one such ban held. It looks at each netblock once
+// at most, and drops nothing when none holds such a ban.
 func (l *Ledger) dropOne() {
-	for {
+	for range l.netblocks.Len() {
 		netblock, bans, _ := l.netblocks.GetOldest()
 
 		i := slices.IndexFunc(*bans, func(ban Ban) bool {