fix(files): ending every session ends every API token, and the share links hold their folder

- A password reset or change, "log out everywhere" and a revoked untrusted password revoke all of the
  account's API tokens too (audited): one minted from a stolen session no longer outlives the answer.
- A bearer token or link grant with a non-ASCII character is refused (401/403), not a 500.
- Making a token's grants or a folder link holds the folder, so a racing delete cannot fail the
  foreign key; a link's view count is one atomic UPDATE.
- A folder link's card counts exactly the files its page lists (one `links.folder_files`).
- A file with no extension is served without a trailing dot.
- 0106's downgrade names a Hub folder so it never collides with one of the user's folders.
- The speed baseline reads share tokens through the app (the database keeps them hashed/encrypted).
- Tests: the token scope on the real session stack, the folder poster, the Hub refused for a link,
  0106's downgrade data step, 0107 without a key; docs/comments follow (three folder kinds, folder
  links, key change, the cleanup job's token warnings).

backend/app/downloads/storage.py

@@ -294,9 +294,13 @@ def served_media_type(media_kind: str | None, container: str | None) -> str | No
 def download_filename(display_name: str, container: str | None, path: Path) -> str:
     """The Content-Disposition filename for a served download: the cleaned display name carrying a single
     correct extension — the asset container, else the file's own suffix — without doubling it when the
-    name already ends in that extension. Shared by the private download and the public watch endpoints."""
+    name already ends in that extension. A file with no extension at all is served without one —
+    never as `name.` (Windows drops a trailing dot, so the saved file would not be the one named).
+    Shared by the private download and the public watch endpoints."""
     ext = container or path.suffix.lstrip(".")
     base = display_filename(display_name)
+    if not ext:
+        return base
     if base.lower().endswith(f".{ext.lower()}"):
         base = base[: -(len(ext) + 1)]
     return f"{base}.{ext}"