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/upload.py

@@ -140,7 +140,7 @@ def ingest_upload(
         if folder_id is not None:
             job.folder_id = folder_id
         if not convert:
-            folders.stamp_expiry(db, job)
+            folders.stamp_done(db, job)
         service.incr_ref_count(db, asset.id)
         db.flush()
         audit.record(db, user_id, AuditAction.FILE_UPLOAD, target_type="file", target_id=job.id)