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_migration_0104.py

@@ -0,0 +1,52 @@
+"""Migration 0104's data step, DB-lane: a done file from before is dated by when it finished where
+its "ready to watch" notice says so, else by when it was asked for; a job not done has no date."""
+
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import text
+
+from app.db import engine
+
+
+def test_a_done_file_takes_its_notice_time_else_its_request_time(db, make_user):
+    uid = make_user().id
+    cfg = Config("alembic.ini")
+    command.downgrade(cfg, "0103_files_sha256")
+    try:
+        with engine.begin() as conn:
+
+            def job(status, created):
+                return conn.execute(
+                    text(
+                        "INSERT INTO download_jobs (user_id, source_kind, source_ref,"
+                        " profile_snapshot, status, created_at) VALUES (:u, 'url', 'x', '{}', :s,"
+                        " CAST(:c AS timestamptz)) RETURNING id"
+                    ),
+                    {"u": uid, "s": status, "c": created},
+                ).scalar_one()
+
+            retried = job("done", "2026-10-01T10:00:00+00:00")
+            quiet = job("done", "2026-10-02T10:00:00+00:00")
+            failed = job("error", "2026-10-03T10:00:00+00:00")
+            for at in ("2026-10-05T09:00:00+00:00", "2026-10-06T09:00:00+00:00"):
+                conn.execute(
+                    text(
+                        "INSERT INTO notifications (user_id, type, title, data, created_at) VALUES"
+                        " (:u, 'download_done', 't', CAST(:d AS json), CAST(:c AS timestamptz))"
+                    ),
+                    {"u": uid, "d": f'{{"job_id": {retried}}}', "c": at},
+                )
+    finally:
+        command.upgrade(cfg, "head")
+
+    with engine.connect() as conn:
+        rows = dict(
+            conn.execute(
+                text(
+                    "SELECT id, to_char(ready_at AT TIME ZONE 'UTC', 'MM-DD HH24') FROM download_jobs"
+                )
+            ).all()
+        )
+    assert rows[retried] == "10-06 09"  # its last finish
+    assert rows[quiet] == "10-02 10"  # no notice left: where it already sat
+    assert rows[failed] is None