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/service/gemini_messages_compat_service.go

@@ -2692,12 +2692,27 @@ func extractGeminiUsage(data []byte) *ClaudeUsage {
 	cand := int(usage.Get("candidatesTokenCount").Int())
 	cached := int(usage.Get("cachedContentTokenCount").Int())
 	thoughts := int(usage.Get("thoughtsTokenCount").Int())
+
+	// 从 candidatesTokensDetails 提取 IMAGE 模态 token 数
+	imageTokens := 0
+	candidateDetails := usage.Get("candidatesTokensDetails")
+	if candidateDetails.Exists() {
+		candidateDetails.ForEach(func(_, detail gjson.Result) bool {
+			if detail.Get("modality").String() == "IMAGE" {
+				imageTokens = int(detail.Get("tokenCount").Int())
+				return false
+			}
+			return true
+		})
+	}
+
 	// 注意：Gemini 的 promptTokenCount 包含 cachedContentTokenCount，
 	// 但 Claude 的 input_tokens 不包含 cache_read_input_tokens，需要减去
 	return &ClaudeUsage{
 		InputTokens:          prompt - cached,
 		OutputTokens:         cand + thoughts,
 		CacheReadInputTokens: cached,
+		ImageOutputTokens:    imageTokens,
 	}
 }