feat: image output token billing, channel-mapped billing source, credits balance precheck

- Parse candidatesTokensDetails from Gemini API to separate image/text output tokens
- Add image_output_tokens and image_output_cost to usage_log (migration 089)
- Support per-image-token pricing via output_cost_per_image_token from model pricing data
- Channel pricing ImageOutputPrice override works in token billing mode
- Auto-fill image_output_price in channel pricing form from model defaults
- Add "channel_mapped" billing model source as new default (migration 088)
- Bills by model name after channel mapping, before account mapping
- Fix channel cache error TTL sign error (115s → 5s)
- Fix Update channel only invalidating new groups, not removed groups
- Fix frontend model_mapping clearing sending undefined instead of {}
- Credits balance precheck via shared AccountUsageService cache before injection
- Skip credits injection for accounts with insufficient balance
- Don't mark credits exhausted for "exhausted your capacity on this model" 429s

backend/internal/handler/admin/channel_handler_test.go

@@ -36,7 +36,7 @@ func TestChannelToResponse_FullChannel(t *testing.T) {
 		RestrictModels:     true,
 		CreatedAt:          now,
 		UpdatedAt:          now.Add(time.Hour),
-		GroupIDs:            []int64{1, 2, 3},
+		GroupIDs:           []int64{1, 2, 3},
 		ModelPricing: []service.ChannelModelPricing{
 			{
 				ID:              10,
@@ -94,8 +94,8 @@ func TestChannelToResponse_EmptyDefaults(t *testing.T) {
 		BillingModelSource: "",
 		CreatedAt:          now,
 		UpdatedAt:          now,
-		GroupIDs:            nil,
-		ModelMapping:        nil,
+		GroupIDs:           nil,
+		ModelMapping:       nil,
 		ModelPricing: []service.ChannelModelPricing{
 			{
 				Platform:    "",
@@ -106,7 +106,7 @@ func TestChannelToResponse_EmptyDefaults(t *testing.T) {
 	}
 
 	resp := channelToResponse(ch)
-	require.Equal(t, "requested", resp.BillingModelSource)
+	require.Equal(t, "channel_mapped", resp.BillingModelSource)
 	require.NotNil(t, resp.GroupIDs)
 	require.Empty(t, resp.GroupIDs)
 	require.NotNil(t, resp.ModelMapping)