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. Only a q missing from the URL is
85. README.md states the range.

Model: opus-5-5

internal/handlers/auth.go

@@ -18,13 +18,14 @@ import (
 	"sneak.berlin/go/pixa/internal/templates"
 )
 
-// errInvalidFormField reports a generator form field whose value is
-// non-numeric or out of range. The offending field name is wrapped in so the
-// response can name it.
+// errInvalidFormField reports a generator form field, or the q parameter of
+// /v1/image/, whose value is non-numeric or out of range. The offending field
+// name is wrapped in so the response can name it.
 var errInvalidFormField = errors.New("invalid")
 
-// Bounds for the generator's quality and ttl fields. maxTTL is in seconds:
-// the expiry calculation time.Duration(ttl) * time.Second overflows above it.
+// Bounds for the generator's quality and ttl fields; the quality bounds also
+// apply to the q parameter of /v1/image/. maxTTL is in seconds: the expiry
+// calculation time.Duration(ttl) * time.Second overflows above it.
 const (
 	minQuality = 1
 	maxQuality = 100
@@ -248,9 +249,9 @@ func parseFormDimension(form url.Values, field string) (int, error) {
 	return value, nil
 }
 
-// parseFormInt reads an optional integer form field, returning def when the
-// field is empty and an error naming the field when the value is non-numeric
-// or outside minValue to maxValue.
+// parseFormInt reads an optional integer form field or URL query parameter,
+// returning def when the field is empty and an error naming the field when the
+// value is non-numeric or outside minValue to maxValue.
 func parseFormInt(
 	form url.Values, field string, def, minValue, maxValue int,
 ) (int, error) {