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/service.py

@@ -205,7 +205,7 @@ def enqueue(
     if asset.status == "ready":
         job.status = JobStatus.DONE
         job.progress = 100
-        folders.stamp_expiry(db, job)
+        folders.stamp_done(db, job)
         asset.last_access_at = datetime.now(timezone.utc)
 
     db.commit()
@@ -285,7 +285,7 @@ def enqueue_edit(
     if asset.status == "ready":  # identical clip already produced for this user → instant
         job.status = JobStatus.DONE
         job.progress = 100
-        folders.stamp_expiry(db, job)
+        folders.stamp_done(db, job)
         asset.last_access_at = datetime.now(timezone.utc)
 
     db.commit()
@@ -374,7 +374,7 @@ def enqueue_clip(
     if asset.status == "ready":  # the same range was already cut for this user → instant
         job.status = JobStatus.DONE
         job.progress = 100
-        folders.stamp_expiry(db, job)
+        folders.stamp_done(db, job)
         asset.last_access_at = datetime.now(timezone.utc)
 
     db.commit()
@@ -399,6 +399,7 @@ def finalize_if_running(db: Session, job_id: int, **fields) -> bool:
         return False
     db.flush()  # asset-then-job lock order: push pending asset writes before we lock the job row
     if fields.get("status") == JobStatus.DONE:
+        fields = {**fields, "ready_at": datetime.now(timezone.utc)}
         # Done is when a file's retention starts: its folder's, unless the user kept it.
         held = db.execute(
             select(DownloadJob.folder_id, DownloadJob.kept).where(DownloadJob.id == job_id)