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/alembic/versions/0104_job_ready_at.py

@@ -0,0 +1,44 @@
+"""A file's "Added" is when it became done
+
+Revision ID: 0104_job_ready_at
+Revises: 0103_files_sha256
+Create Date: 2026-10-12
+
+`download_jobs.ready_at` is when a job became done, and the Files library sorts and dates by it: a
+download that failed for days and then went through used to sit days down the list, under its
+request date. Every path to done stamps it, held by `ck_download_jobs_ready_at`.
+
+A done job from before has no record of that moment, except where its "ready to watch" notice is
+still in the owner's inbox (read notices are pruned past a cap): that notice was written the moment
+it finished. The rest take the request date, which is where they already sat.
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0104_job_ready_at"
+down_revision: str | None = "0103_files_sha256"
+branch_labels: str | None = None
+depends_on: str | None = None
+
+
+def upgrade() -> None:
+    op.add_column("download_jobs", sa.Column("ready_at", sa.DateTime(timezone=True)))
+    op.execute(
+        """
+        UPDATE download_jobs j SET ready_at = COALESCE(
+            (SELECT max(n.created_at) FROM notifications n
+              WHERE n.type = 'download_done' AND n.user_id = j.user_id
+                AND n.data->>'job_id' = j.id::text),
+            j.created_at)
+        WHERE j.status = 'done'
+        """
+    )
+    op.create_check_constraint(
+        "ck_download_jobs_ready_at", "download_jobs", "status <> 'done' OR ready_at IS NOT NULL"
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_download_jobs_ready_at", "download_jobs", type_="check")
+    op.drop_column("download_jobs", "ready_at")