Refuse an empty fit on /v1/image/ with 400 (closes #139)

A fit in the URL with an empty value (fit=) was treated as missing, so
it was served as cover and verified against a signature made for cover.
It is now a 400 naming fit, the same rule the route applies to an empty
q. It is checked before the existing fit-mode check, which takes an
empty fit as missing; any other value still goes through that check
unchanged. Only a fit missing from the URL is cover.

Model: opus-5-5

README.md

@@ -132,7 +132,8 @@ Where:
   or `85` when the URL has no `q`; a request whose `q` is anything else is
   refused with 400
 - `fit` — the URL's `fit` query parameter (cover, contain, fill, inside,
-  outside), or `cover` when the URL has no `fit`
+  outside), or `cover` when the URL has no `fit`; a request whose `fit` is
+  anything else, an empty `fit=` included, is refused with 400
 
 **Example:** resize `https://cdn.example.com/photos/cat.jpg` to 800x600
 WebP with expiration 1704067200, default quality and fit: