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/downloads/folders.py

@@ -16,7 +16,7 @@ file in the TikTok folder would be a lie the folder's name tells.
 Retention is the folder's: a source folder keeps a file `download_retention_days` (the value when the
 folder was made; the user can change it), Uploads and own folders keep it until it is deleted. A
 done file carries its own `expires_at`, stamped by `done_expiry` when it finishes — unless the user
-kept it.
+kept it — and its `ready_at`, the moment it finished (`stamp_done`).
 """
 
 from datetime import datetime, timedelta, timezone
@@ -136,8 +136,9 @@ def done_expiry(db: Session, folder_id: int | None, kept: bool) -> datetime | No
     return None if days is None else datetime.now(timezone.utc) + timedelta(days=days)
 
 
-def stamp_expiry(db: Session, job: DownloadJob) -> None:
-    """Stamp a job that just became done (in this session) with its expiry."""
+def stamp_done(db: Session, job: DownloadJob) -> None:
+    """Stamp a job that just became done (in this session): when it did, and its expiry."""
+    job.ready_at = datetime.now(timezone.utc)
     job.expires_at = done_expiry(db, job.folder_id, job.kept)
     job.expiry_warned = False
 
@@ -163,7 +164,7 @@ def upload_target(db: Session, user_id: int, folder_id: int) -> FileFolder:
 
 def restamp(db: Session, job: DownloadJob) -> None:
     """Start a file's clock again from now: it moved to another folder, or its user stopped keeping
-    it. A file not done yet gets its expiry when it finishes (`stamp_expiry`), so it has none now."""
+    it. A file not done yet gets its expiry when it finishes (`stamp_done`), so it has none now."""
     done = job.status == JobStatus.DONE
     job.expires_at = done_expiry(db, job.folder_id, job.kept) if done else None
     job.expiry_warned = False