fix(files): a file is added when it is done, not when it was asked for

A download that failed for days and then went through sat days down the library, under its
request date, and read as missing. A job now records when it became done (`ready_at`), stamped
where the expiry already is (`folders.stamp_done`, `finalize_if_running`) and on an insert that is
already done; a check constraint holds every path to done to it. The library sorts and dates its
"Added" by it; the queue keeps the request order.

The migration dates a file from before by its "ready to watch" notice where the inbox still holds
one, else by its request date, where it already sat.

frontend/src/components/Files.tsx

@@ -402,9 +402,9 @@ export default function Files() {
       align: "right",
       nowrap: true,
       sortable: true,
-      sortValue: (f) => f.shared_at ?? f.created_at ?? "",
+      sortValue: (f) => f.shared_at ?? f.ready_at ?? f.created_at ?? "",
       render: (f) => {
-        const at = f.shared_at ?? f.created_at;
+        const at = f.shared_at ?? f.ready_at ?? f.created_at;
         return (
           <span className="text-xs text-muted" title={formatDateTime(at, lang)}>
             {relativeTime(at, lang, t)}