Refuse SVG at the fetcher and never save an unknown input as JPEG (part of #68)

The fetcher no longer accepts image/svg+xml, so an SVG source gets the
refusal every unaccepted upstream type gets, which both image routes now
answer with 415 where they answered 500. orig of an input the processor
cannot name is ErrUnsupportedInputFormat, also answered 415, where it
was saved as JPEG. README.md drops SVG from the source formats and lists
the 415.

Model: opus-5-5

internal/handlers/image.go

@@ -247,6 +247,14 @@ func (s *Handlers) respondImageError(
 		return
 	}
 
+	if errors.Is(err, httpfetcher.ErrInvalidContentType) ||
+		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat) {
+		s.respondError(w, "unsupported source format",
+			http.StatusUnsupportedMediaType)
+
+		return
+	}
+
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }
 

internal/handlers/imageenc.go

@@ -161,6 +161,10 @@ func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 		errors.Is(err, imageprocessor.ErrTooManyImages):
 		s.respondError(w, "server busy, try again later",
 			http.StatusServiceUnavailable)
+	case errors.Is(err, httpfetcher.ErrInvalidContentType),
+		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat):
+		s.respondError(w, "unsupported source format",
+			http.StatusUnsupportedMediaType)
 	default:
 		s.log.Error("image request failed", "error", err)
 		s.respondError(w, "internal error", http.StatusInternalServerError)