Offer archive expiry choices on the target forms, show plain units (closes #396)

A database target's archive expiry was typed by hand as never or a raw duration such as 720h, and the target list showed it back raw. Adding or editing a database target now offers the new-webhook page's list of choices (never, 1h, 12h, 24h, 30d, 90d, 365d), defined once and shared by all three forms. The edit form starts on the stored expiry, or on the submitted one after a refused save; a stored value outside the choices is listed under its own value, so saving unchanged keeps it. The target list shows the expiry in plain units: "30 days", "12 hours", "never".

Model: opus-5-5

internal/handlers/source_management.go

@@ -312,6 +312,9 @@ func newSourceFormData(
 		tmplKeyError:           errMsg,
 		"Form":                 in,
 		"DefaultRetentionDays": database.DefaultRetentionDays,
+		tmplKeyArchiveExpiryChoices: archiveExpiryOptions(
+			in.ArchiveExpiry,
+		),
 	}
 }
 
@@ -623,6 +626,9 @@ func (h *Handlers) renderSourceDetail(
 		"Stats":           h.loadWebhookStats(webhook.ID, entrypoints, targets),
 		tmplKeyTargetForm: targetForm,
 		"TargetError":     targetErr,
+		// The add target form's select starts on its expiry
+		// through Alpine, so no choice is selected here.
+		tmplKeyArchiveExpiryChoices: archiveExpiryChoices(),
 	}
 
 	status := http.StatusOK