Refuse an empty fit on /v1/image/ with 400 (closes #139)

A fit in the URL with an empty value (fit=) was treated as missing, so
it was served as cover and verified against a signature made for cover.
It is now a 400 naming fit, the same rule the route applies to an empty
q. It is checked before the existing fit-mode check, which takes an
empty fit as missing; any other value still goes through that check
unchanged. Only a fit missing from the URL is cover.

Model: opus-5-5

internal/handlers/handlers_internal_test.go

@@ -356,3 +356,41 @@ func TestHandleImage_InvalidQuery_Returns400(t *testing.T) {
 		})
 	}
 }
+
+// TestHandleImage_EmptyFit_Returns400 verifies that the plain image route
+// answers a fit that is in the URL but empty with 400 naming fit, instead of
+// serving the image as cover. Only a fit missing from the URL means cover.
+func TestHandleImage_EmptyFit_Returns400(t *testing.T) {
+	t.Parallel()
+
+	fix := setupTestHandler(t)
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", fix.handler.HandleImage())
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+		"/v1/image/"+fix.goodHost+"/images/photo.jpg/50x50.jpeg?fit=", nil)
+	rec := httptest.NewRecorder()
+
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+
+	t.Logf("GET %s: %d %s", req.URL, rec.Code, rec.Body)
+
+	var body struct {
+		Error string `json:"error"`
+	}
+
+	err := json.NewDecoder(rec.Body).Decode(&body)
+	if err != nil {
+		t.Fatalf("decoding response body: %v", err)
+	}
+
+	wantError := `invalid fit: not a fit mode, got ""`
+	if body.Error != wantError {
+		t.Errorf("error = %q, want %q", body.Error, wantError)
+	}
+}