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/delivery/target_headers_test.go

@@ -216,8 +216,9 @@ func TestNewTargetConfigForm(t *testing.T) {
 	assert.Empty(t, form.URL)
 }
 
-// A keep-forever archive target must pre-fill as an empty field, so
-// saving the form back unchanged stores the same empty config.
+// A keep-forever archive target yields an empty expiry, so the edit
+// form starts on never; saving it unchanged stores never, which means
+// the same as an empty expiry.
 func TestNewTargetConfigForm_DatabaseNeverIsBlank(t *testing.T) {
 	t.Parallel()