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/handlers/image.go

@@ -255,6 +255,14 @@ func (s *Handlers) respondImageError(
 		return
 	}
 
+	if errors.Is(err, imageprocessor.ErrOutputTooLarge) {
+		s.respondError(w, fmt.Sprintf("size too large for the source: the side "+
+			"computed from its proportions is above %d",
+			imageprocessor.MaxOutputDimension), http.StatusBadRequest)
+
+		return
+	}
+
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }