Refuse a q outside 1-100 on /v1/image/ with 400 (closes #134)

The route ignored a q that was not a number or was outside 1-100 and
used 85, so q=banana or q=500 was served as if q were absent and
verified against a signature made for 85.

It now reads q with the check the URL generator uses for its quality
field (parseFormInt with minQuality and maxQuality, default
encurl.DefaultQuality) and answers anything else with a 400 naming q
and the value. That check takes an empty value as missing, so an empty
q in the URL is refused before it. The query string is read with
url.ParseQuery, since r.URL.Query() drops a pair it cannot decode, such
as q=80%; one that cannot be decoded is a 400 showing it. Only a q
missing from the URL is 85. README.md states the range.

Model: opus-5-5

README.md

@@ -125,8 +125,9 @@ Where:
 - `height` — requested height in pixels, `0` for original
 - `format` — output format (jpeg, png, webp, avif, gif, orig)
 - `expiration` — Unix timestamp when signature expires
-- `quality` — the URL's `q` query parameter (1-100), or `85` when the URL
-  has no `q`
+- `quality` — the URL's `q` query parameter, a whole number from 1 to 100,
+  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`