Refuse a size whose computed side exceeds the maximum (closes #241)

A side given as 0 is computed from the source's proportions, and
nothing held it to 8192: 0x8192 of a 3000x600 source was resized to
40960x8192 and encoded. Process now refuses such a size with
ErrOutputTooLarge once it knows the source's dimensions, before the
resize and the encode, and both image routes answer it 400 with the
JSON error body. It is a processing error, not a failed fetch, so it
is never negative-cached. imgcache.MaxDimension is now
imageprocessor.MaxOutputDimension, so the request limit and the output
limit are one value. README.md says it under Routes and Encrypted URLs.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -7,6 +7,8 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 )
 
 // URL parsing errors.
@@ -20,8 +22,9 @@ var (
 	ErrPathTraversal     = errors.New("path traversal detected")
 )
 
-// MaxDimension is the maximum allowed width or height.
-const MaxDimension = 8192
+// MaxDimension is the maximum allowed width or height, the same as the image
+// processor's maximum for an output image.
+const MaxDimension = imageprocessor.MaxOutputDimension
 
 // ValidateDimension checks one requested width or height; name ("width" or
 // "height") appears in the error. 0 means "original size" and is valid.