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_files_api.py

@@ -59,7 +59,7 @@ def _file(db, user_id, *, kind="youtube", ref=None, title="A title", size=10, re
     db.add(job)
     db.flush()
     folders.place(db, job)
-    folders.stamp_expiry(db, job)
+    folders.stamp_done(db, job)
     service.incr_ref_count(db, asset.id)
     db.commit()
     return job
@@ -245,6 +245,38 @@ def test_the_list_sorts_by_name_size_or_when_added(db, make_user, as_user):
     assert ids(sort="size", order="desc") == [b.id, z.id, a.id]
 
 
+def test_a_file_is_added_when_it_is_done_not_when_it_was_asked_for(db, make_user, as_user):
+    """A download that failed for days and then went through is new: it lists first, dated today."""
+    alice = make_user()
+    early = _file(db, alice.id, created_at=NOW - timedelta(days=1))
+    early.ready_at = NOW - timedelta(days=1)
+    late = _file(db, alice.id, status=JobStatus.RUNNING, created_at=NOW - timedelta(days=5))
+    late.ready_at = None
+    db.commit()
+    assert service.finalize_if_running(db, late.id, status=JobStatus.DONE)
+    db.commit()
+    db.expire_all()  # the finalize is a Core UPDATE; the route reads this same session
+    got = as_user(alice).get("/api/files").json()
+    assert [f["id"] for f in got] == [late.id, early.id]
+    ready = datetime.fromisoformat(got[0]["ready_at"])
+    assert ready > NOW - timedelta(minutes=5)
+    assert datetime.fromisoformat(got[0]["created_at"]) < NOW - timedelta(days=4)
+
+
+def test_a_job_made_done_is_ready_as_it_is_made(db, make_user):
+    # An upload kept as it came is inserted done; the database refuses a done job without the moment.
+    job = DownloadJob(
+        user_id=make_user().id,
+        source_kind="upload",
+        source_ref="r",
+        profile_snapshot={},
+        status=JobStatus.DONE,
+    )
+    db.add(job)
+    db.commit()
+    assert job.ready_at is not None
+
+
 def test_shared_with_me_lists_what_others_shared_and_nothing_else(db, make_user, as_user):
     alice, bob, carol = make_user(), make_user(), make_user()
     theirs = _file(db, bob.id)