fix(billing): channel pricing override for image generation + display image_output_tokens

[touwaeriol]

背景 / Background

按 token 计费的图片生成模型（如 gemini-3.x image-preview）存在两个相互配套的问题：

1. 后端定价：当渠道配置 token 定价（含图片输出价）时，ImageOutputPricePerToken 未真正覆盖默认值——若渠道显式配置为 0，会回退到 LiteLLM 或 group config 的价格；同时区间定价 fallback 到 BasePricing 时图片价格也漏覆盖。
2. 前端展示：按 token 计费的图片请求在用量页仍按"1 张 (2K)"展示，API 响应缺 image_output_tokens / image_output_cost 字段，前端无法显示 token 拆分。

Image generation models billed per token (e.g. gemini-3.x image-preview) had two related issues:

1. Backend pricing: When a channel configures token pricing (including image output price), ImageOutputPricePerToken was not fully overriding the default sources — explicit zero on the channel would fall back to LiteLLM/group config; interval fallback to BasePricing also missed the image price.
2. Frontend display: Token-billed image requests still showed as "1 image (2K)" on usage pages, and the API response was missing image_output_tokens / image_output_cost fields needed for the breakdown.

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目的 / Purpose

让渠道定价完全主导按 token 计费的图片生成计费，并在用量页面正确展示 token / 费用拆分。

Make channel pricing the single source of truth for token-billed image generation, and properly display the token/cost breakdown on usage pages.

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改动内容 / Changes

后端 / Backend

• 新增 ImageOutputPriceExplicit 标志（billing_service.go）：区分"渠道显式配置图片输出价"和"未配置缺失值"。true 时即使价格为 0 也不回退到 outputPrice
• GetModelPricingWithChannel：渠道存在时，未配置的 ImageOutputPrice 归零并设 Explicit=true（不再回退 LiteLLM）
• computeTokenBreakdown：用 Explicit 标志决定是否回退 outputPrice
• model_pricing_resolver.go：applyTokenOverrides flat / interval 两条路径都设 explicit + 归零；intervalToModelPricing 也接收 channel pricing 设 explicit
• calculateRecordUsageCost / OpenAI calculateOpenAIRecordUsageCost：渠道定价为 token 模式时图片走 token 路径而非按图计费
• buildRecordUsageLog + OpenAI RecordUsage：图片请求走 token 计费时 RateMultiplier 用普通 multiplier 而非 image multiplier
• DTO 补字段：UsageLog DTO 添加 image_output_tokens 和 image_output_cost（数据库/service 早有，DTO 漏映射）

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• ImageOutputPriceExplicit flag (billing_service.go): Distinguish "channel explicitly set image output price" from "missing value". When true, a zero price does NOT fall back to outputPrice
• GetModelPricingWithChannel: With a channel present, missing ImageOutputPrice is zeroed and marked Explicit=true (no more LiteLLM fallback)
• computeTokenBreakdown: Uses the Explicit flag to decide whether to fall back to outputPrice
• model_pricing_resolver.go: Both flat and interval paths in applyTokenOverrides set explicit + zero; intervalToModelPricing also receives channel pricing and sets explicit
• calculateRecordUsageCost / OpenAI calculateOpenAIRecordUsageCost: When channel pricing is token-mode, image requests go through the token path instead of per-image billing
• buildRecordUsageLog + OpenAI RecordUsage: When a token-billed image request is recorded, RateMultiplier uses the regular multiplier rather than the image multiplier
• DTO field addition: Add image_output_tokens and image_output_cost to UsageLog DTO (already existed in DB/service, missing in DTO mapping)

前端 / Frontend

• 计费模式感知：isImageUsage() 现在检查 billing_mode !== 'token'，token 计费的图片请求走 token 展示路径而非图片格式
• Token 明细 Tooltip：拆分"输出 Token"为"输出 Token"（文本部分）+ "图片输出 Token"（粉色高亮），纯图片输出时隐藏文本行
• 费用 Tooltip：新增图片输出成本行和图片输出单价（每百万 Token）
• Token 列：按量计费的图片请求显示完整 token 明细，含粉色图片图标
• 共享工具函数：isImageUsage、hasImageOutputTokens、textOutputTokens 抽到 @/utils/billingMode.ts 和 @/utils/imageUsage.ts，Admin 和 User 页面共用
• i18n：imageOutput* 翻译统一到 usage.* 命名空间

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• Billing mode awareness: isImageUsage() now checks billing_mode !== 'token', routing token-billed image requests to the token display path
• Token tooltip: Split "Output Tokens" into text output + "Image Output Tokens" (pink highlight); hide text row for pure-image output
• Cost tooltip: Add image output cost row and image output unit price (per 1M tokens)
• Token column: Show full token breakdown for token-billed image requests with a pink image icon
• Shared utilities: Extract isImageUsage, hasImageOutputTokens, textOutputTokens to shared utils used by both Admin and User pages
• i18n: Unify imageOutput* translation keys under usage.* namespace

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测试 / Tests

后端新增 6 个单测覆盖：

• TestGetModelPricingWithChannel_NilImageOutputPriceZerosAndMarksExplicit
• TestComputeTokenBreakdown_ExplicitZeroImagePrice_NoFallback
• TestComputeTokenBreakdown_NonExplicitZeroImagePrice_FallsBackToOutput
• TestApplyTokenOverrides_FlatSetsImageOutputPriceExplicit
• TestApplyTokenOverrides_FlatWithImageOutputPriceSetsExplicit
• TestApplyTokenOverrides_IntervalSetsImageOutputPriceExplicit

全部 PASS。

Six new backend unit tests covering all four code paths (flat / interval / non-explicit fallback / explicit zero). All passing.

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截图 / Screenshot

Token 列展示（按量计费的图片生成）：

↓ 24  ↑ 1,514
🖼 1,120            ← 图片输出 Token（粉色）

Token 明细 Tooltip：

输入 Token         21
图片输出 Token   1,120    ← 粉色，与"输出 Token"区分
总 Token         1,141

费用 Tooltip：

输入成本         \$0.000012
图片输出成本     \$0.067200    ← 新增行
输入单价         \$0.5000 / 1M Token
图片输出单价     \$60.0000 / 1M Token  ← 新增行

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备注 / Notes

This supersedes #2774 by bundling the backend billing fix with the frontend display changes; the prior PR's frontend-only scope depended on backend changes that hadn't landed.