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/image.go

@@ -2,12 +2,14 @@ package handlers
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"strconv"
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
@@ -100,23 +102,31 @@ func (s *Handlers) parseImageRequest(
 		}
 	}
 
-	// Parse optional quality and fit params
-	if qStr := query.Get("q"); qStr != "" {
-		q, parseErr := strconv.Atoi(qStr)
-		if parseErr == nil && q > 0 && q <= 100 {
-			req.Quality = q
-		}
+	// Parse optional quality and fit params. Only a q missing from the URL is
+	// 85. A q in the URL that is not a whole number from 1 to 100, an empty
+	// one included, is refused, checked as the generator checks its quality
+	// field; that check alone would take an empty q as missing.
+	qStr := query.Get("q")
+	if query.Has("q") && qStr == "" {
+		s.respondError(w, `invalid q: not a number, got ""`,
+			http.StatusBadRequest)
+
+		return nil, false
+	}
+
+	req.Quality, err = parseFormInt(query, "q",
+		encurl.DefaultQuality, minQuality, maxQuality)
+	if err != nil {
+		s.respondError(w, fmt.Sprintf("%v, got %q", err, qStr),
+			http.StatusBadRequest)
+
+		return nil, false
 	}
 
 	if fit := query.Get("fit"); fit != "" {
 		req.FitMode = imgcache.FitMode(fit)
 	}
 
-	// Default quality if not set
-	if req.Quality == 0 {
-		req.Quality = 85
-	}
-
 	// Default fit mode if not set
 	if req.FitMode == "" {
 		req.FitMode = imgcache.FitCover