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_clip.py

@@ -374,7 +374,7 @@ def test_a_clip_is_not_redownloadable(db, make_user, as_user, yt_video):
     c = as_user(make_user())
     job = _clip(c, source="youtube", ref=yt_video).json()
     row = db.get(DownloadJob, job["id"])
-    row.status = "done"
+    row.status, row.ready_at = "done", row.created_at
     db.commit()
     r = c.post(f"/api/downloads/{job['id']}/redownload")
     assert (r.status_code, r.json()["detail"]["reason"]) == (409, "not_redownloadable")