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/scheduler.py

@@ -370,7 +370,8 @@ def _explore_cleanup_job() -> None:
 
 
 def _download_gc_job() -> None:
-    # Retention GC for the download center: pre-expiry warnings, TTL deletion, LRU eviction.
+    # Retention GC for the download center: pre-expiry warnings, TTL deletion, LRU eviction — and
+    # the week-ahead warning before an API token runs out (`apitokens.warn_expiring`).
     # Pure disk/DB work (no YouTube quota).
     _job("download_gc", run_download_gc)