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.

backend/app/routes/files.py

@@ -78,6 +78,7 @@ def serialize(job: DownloadJob, asset: MediaAsset | None) -> dict:
         "error_detail": job.error_detail,
         "queue_pos": job.queue_pos,
         "created_at": iso(job.created_at),
+        "ready_at": iso(job.ready_at),
         "source_kind": job.source_kind,
         "source_ref": job.source_ref,
         "source_url": service.reference_url(job, asset),
@@ -545,10 +546,10 @@ def list_files(
                 func.coalesce(DownloadJob.display_uploader, MediaAsset.uploader, "").ilike(like),
             )
         )
-    # A file shared with the caller was added to THEIR list when it was shared, not when its owner
-    # downloaded it.
+    # A file is added to its owner's list when it is done, not when it was asked for (a download that
+    # failed for days and then went through is new); one shared with the caller, when it was shared.
     key = {
-        "added": DownloadShare.created_at if shared else DownloadJob.created_at,
+        "added": DownloadShare.created_at if shared else DownloadJob.ready_at,
         "name": collated(name, lang_for_user(user)),
         "size": MediaAsset.size_bytes,
     }[sort]