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

Adding or editing a database target now offers the archive expiry
choices of the new webhook page (never, 1h, 12h, 24h, 30d, 90d, 365d)
in place of a text field. The list is defined once, in
internal/handlers/archive_expiry.go, and all three forms render it. The
edit form starts on the expiry it shows: the stored one, or the
submitted one after a refused save. One that is not among the choices
is listed first as its own entry, so saving unchanged keeps it. The
target list shows the expiry in plain units, such as "30 days" or
"12 hours", or "never".

Model: opus-5-5

internal/handlers/target_edit_test.go

@@ -613,12 +613,16 @@ func TestHandleTargetEditSubmit_RefusedFormComesBack(t *testing.T) {
 			page := w.Body.String()
 			assert.Contains(t, page, `class="alert-error">`+tc.reason)
 
-			// headers is the form's one textarea; every other field is
-			// an input.
+			// headers is the form's one textarea and expiry its one
+			// select; every other field is an input.
 			for field := range form {
 				shown := `name="` + field + `" value="` + form.Get(field) + `"`
-				if field == "headers" {
+
+				switch field {
+				case "headers":
 					shown = ">" + form.Get(field) + "</textarea>"
+				case "expiry":
+					shown = `<option value="` + form.Get(field) + `" selected>`
 				}
 
 				assert.Contains(t, page, shown)