Store a webhook's retention as a plain day count, 0 meaning forever (closes #539)

retention_days loses its column default of 30, and Webhook.RetentionDays
its default tag, so a 0 is stored as 0. The 365000-day stand-in
RetentionForeverDays and the BeforeSave hook that wrote it are gone. The
reaper and the "forever" label read 0 as forever, and the forms accept 0
or 1 to MaxFiniteRetentionDays; a blank field still gives 30 on the new
webhook form and keeps the stored value on the edit form. A negative
value, which the forms refuse, is still read as forever so it cannot put
the reaper's cutoff in the future.

Model: opus-5-5

internal/handlers/shared.go

@@ -16,21 +16,16 @@ import (
 //
 // An empty value yields fallback, which lets the create path apply the
 // default and the edit path leave the stored value unchanged. A value
-// of 0 is returned as 0 and is rewritten to the retain-forever
-// sentinel by database.Webhook's BeforeSave hook. Anything unparseable
-// or negative is refused rather than silently given a default.
+// of 0 means retain events forever and is returned as it is. Anything
+// unparseable or negative is refused rather than silently given a
+// default.
 //
 // The upper bound is not cosmetic. The reaper computes its cutoff as a
 // time.Duration, an int64 nanosecond count, so a day count above
 // database.MaxFiniteRetentionDays overflows, puts the cutoff in the
-// future, and deletes every event the webhook has. A finite value
-// above that ceiling is therefore refused, and the message names the
-// ceiling rather than implying the input was not a number.
-//
-// A value at or above the retain-forever sentinel is not out of range:
-// it is what the edit form pre-fills for a retain-forever webhook, so
-// submitting the form back unchanged has to keep meaning "forever"
-// rather than being rejected.
+// future, and deletes every event the webhook has. A value above that
+// ceiling is therefore refused, and the message names the ceiling
+// rather than implying the input was not a number.
 func parseRetentionDays(raw string, fallback int) (int, string) {
 	raw = strings.TrimSpace(raw)
 	if raw == "" {
@@ -43,10 +38,6 @@ func parseRetentionDays(raw string, fallback int) (int, string) {
 			"retain events forever."
 	}
 
-	if v >= database.RetentionForeverDays {
-		return database.RetentionForeverDays, ""
-	}
-
 	if v > database.MaxFiniteRetentionDays {
 		return 0, "Retention must be at most " +
 			strconv.Itoa(database.MaxFiniteRetentionDays) +