Add GET /api/v1/character/details with each image's change time

Returns every character image with when it was last changed, in Unix seconds,
so clients can cache the images and download only the ones that changed.

internal/server/handlers_test.go

@@ -167,6 +167,16 @@ func TestCharacterEndpoints(t *testing.T) {
 	require.NoError(t, json.Unmarshal(e.do(http.MethodGet, "/api/v1/character/list").Body.Bytes(), &list))
 	assert.Equal(t, []string{"Adol.png", "Aloy.jpg"}, list)
 
+	var details []struct {
+		Name     string `json:"name"`
+		Modified int64  `json:"modified"`
+	}
+	require.NoError(t, json.Unmarshal(e.do(http.MethodGet, "/api/v1/character/details").Body.Bytes(), &details))
+	require.Len(t, details, 2)
+	assert.Equal(t, "Adol.png", details[0].Name)
+	assert.Equal(t, "Aloy.jpg", details[1].Name)
+	assert.Positive(t, details[0].Modified)
+
 	rec := e.do(http.MethodGet, "/api/v1/character?name=Adol.png")
 	assert.Equal(t, http.StatusOK, rec.Code)
 	assert.Equal(t, "image/png", rec.Header().Get("Content-Type"))