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/channel.go

@@ -24,8 +24,9 @@ func (m BillingMode) IsValid() bool {
 }
 
 const (
-	BillingModelSourceRequested = "requested"
-	BillingModelSourceUpstream  = "upstream"
+	BillingModelSourceRequested     = "requested"
+	BillingModelSourceUpstream      = "upstream"
+	BillingModelSourceChannelMapped = "channel_mapped"
 )
 
 // Channel 渠道实体
@@ -34,7 +35,7 @@ type Channel struct {
 	Name               string
 	Description        string
 	Status             string
-	BillingModelSource string // "requested" or "upstream"
+	BillingModelSource string // "requested", "upstream", or "channel_mapped"
 	RestrictModels     bool   // 是否限制模型（仅允许定价列表中的模型）
 	CreatedAt          time.Time
 	UpdatedAt          time.Time
@@ -180,6 +181,7 @@ func (c *Channel) Clone() *Channel {
 type ChannelUsageFields struct {
 	ChannelID          int64  // 渠道 ID（0 = 无渠道）
 	OriginalModel      string // 用户原始请求模型（渠道映射前）
-	BillingModelSource string // 计费模型来源："requested" / "upstream"
+	ChannelMappedModel string // 渠道映射后的模型名（无映射时等于 OriginalModel）
+	BillingModelSource string // 计费模型来源："requested" / "upstream" / "channel_mapped"
 	ModelMappingChain  string // 映射链描述，如 "a→b→c"
 }