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. An absent or empty q is still 85. README.md states the
range.

Model: opus-5-5

TODO.md

@@ -30,6 +30,12 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 refuse an invalid `q` on `/v1/image/` (closes #134): a `q`
+  that is not a whole number from 1 to 100 is a 400 naming `q` and the
+  value, instead of being served at the default 85; the route reads `q`
+  with the generator's quality check (`parseFormInt` with `minQuality`
+  and `maxQuality`); an absent or empty `q` is still 85; `README.md`
+  states the range.
 - 2026-09-28 unknown `PIXA_` environment variables abort startup (closes
   #133): a variable whose name starts with `PIXA_` but is neither a
   setting's variable nor `PIXA_CONFIG_PATH` aborts startup naming it, as