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/tests/test_upload.py

@@ -320,7 +320,7 @@ def test_upload_route_media_html_extension_not_served_as_html(
     assert job["status"] == "queued"
     row = db.get(DownloadJob, job["id"])
     db.get(MediaAsset, row.asset_id).status = "ready"
-    row.status = "done"
+    row.status, row.ready_at = "done", row.created_at
     db.commit()
 
     # Authenticated endpoint: never text/html.