fix(files): review round 3, the server — a fetched locked re-read, a finish that follows its file, an eviction that counts again

- set_retention and delete_own re-read the folder under its lock and use that row (an unfetched ORM
  result never refreshes the object); a folder gone meanwhile is refused (409), and the route audits
  the retention actually replaced
- a finishing download is stamped only while it is still in the folder (and keep) it was read with:
  a move or a folder delete landing meanwhile sends the read round again, so no file is done with no
  expiry
- the eviction pass counts the disk before each candidate and skips one asked for since it began
- docs: source builds get their own volume-copy snippet
- race tests assert the lock wait; one built on a committing delete never raced

docs/self-hosting.md

@@ -279,8 +279,18 @@ that name. To move to the new layout, download the new
   Open the Files library and check your files play; only then remove the old volume
   (`docker volume rm "$OLD"`).
 - **If you build from source**, a `git pull` brings the new `docker-compose.yml`, whose default
-  volume is `files` (it was `downloads`): copy it over the same way, with `downloads` and `files` in
-  place of `siftlode_downloads` and `siftlode_files`, before you start the new version.
+  volume is `files` (it was `downloads`). Copy it over before you start the new version — with your
+  own compose file, not the self-host one above, whose volumes are other ones:
+
+  ```bash
+  docker compose down
+  docker volume ls --format '{{.Name}}' | grep '_downloads$'   # e.g. siftlode_downloads
+  OLD=siftlode_downloads                                         # the name it printed
+  NEW="${OLD%downloads}files"
+  docker volume create "$NEW"
+  docker run --rm -v "$OLD":/from -v "$NEW":/to alpine cp -a /from/. /to/
+  docker compose up -d --build
+  ```
 
 If the files library is not mounted where the app looks, the API says so in its log at start
 (`The files library is not reachable`), and the download worker refuses to start rather than write