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/tests/test_links.py

@@ -53,6 +53,9 @@ class TestGrant:
     def test_malformed_grants_are_rejected_not_crashed(self):
         for bad in [None, "", "no-dot", "notanumber.sig"]:
             assert check_grant("tok", bad, 0) is False
+        # The query string decodes as UTF-8: a signature past ASCII is just a wrong one.
+        future = int(datetime.now(timezone.utc).timestamp()) + _GRANT_TTL
+        assert check_grant("tok", f"{future}.é", 0) is False
 
 
 def _asset(**over):