feat(channel): add channel management with custom model pricing

[touwaeriol]

背景 / Background

目前系统缺乏对不同用户分组进行差异化定价的能力。管理员无法为不同分组配置独立的模型定价、计费模式或模型访问限制，所有分组共享相同的全局定价策略。

Currently the system lacks the ability to apply differentiated pricing to different user groups. Administrators cannot configure independent model pricing, billing modes, or model access restrictions per group — all groups share the same global pricing strategy.

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目的 / Purpose

引入「渠道(Channel)」管理功能，允许管理员创建渠道并关联分组，为每个渠道独立配置模型定价规则、模型映射和访问限制，实现灵活的分组级计费管理。

Introduce a "Channel" management feature that allows administrators to create channels associated with groups, independently configure model pricing rules, model mappings, and access restrictions per channel — enabling flexible group-level billing management.

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改动内容 / Changes

后端 / Backend

• 渠道 CRUD：完整的 Handler → Service → Repository 三层架构，支持创建、查询、更新、删除渠道
• 数据库迁移：7 个 migration 文件（channels 主表、channel_groups 关联表、channel_model_pricing 定价表、channel_pricing_intervals 分层定价子表）
• 三种计费模式：Token 计费、按次计费（Per Request）、图片计费（Image）
• 平台级配置：每个渠道可按平台（Anthropic/OpenAI/Gemini 等）独立配置定价和映射
• 模型映射：渠道级模型名称转换（请求模型 → 上游模型），在账号级映射之前执行
• 计费基准控制：billing_model_source 支持 requested / upstream / channel_mapped 三种模式
• 模型限制：restrict_models 开关，限制分组只能访问渠道定义的模型列表
• 分组隔离：每个分组最多属于一个渠道，防止定价冲突
• 通配符匹配：模型名称支持通配符前缀匹配
• 上下文区间定价：基于 input token 数量分层定价
• ImageOutputPricePerToken：ModelPricing 新增图片输出 token 定价字段

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• Channel CRUD: Complete Handler → Service → Repository layers with full create, read, update, delete support
• Database migrations: 7 migration files (channels, channel_groups, channel_model_pricing, channel_pricing_intervals)
• Three billing modes: Token billing, per-request billing, and image billing
• Platform-level config: Per-platform (Anthropic/OpenAI/Gemini etc.) pricing and mapping within each channel
• Model mapping: Channel-level model name translation (request → upstream), runs before account-level mapping
• Billing model source: Supports requested / upstream / channel_mapped modes
• Model restriction: restrict_models toggle to limit groups to channel-defined models only
• Group isolation: Each group belongs to at most one channel to prevent pricing conflicts
• Wildcard matching: Model names support wildcard prefix matching
• Context interval pricing: Tiered pricing based on input token count
• ImageOutputPricePerToken: New field in ModelPricing for image output token pricing

前端 / Frontend

• 渠道管理页面（ChannelsView.vue, 1065 行）：表格列表、搜索、状态切换、创建/编辑对话框
• 定价卡片组件（PricingEntryCard.vue）：支持 Token/按次/图片三种模式切换、区间配置
• 模型标签输入（ModelTagInput.vue）：支持回车添加、粘贴批量导入
• 区间行组件（IntervalRow.vue）：上下文区间定价配置
• 侧边栏导航：新增渠道管理入口
• 路由注册：/admin/channels 路由
• 完整 i18n：中英文翻译（102 行/语言）

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• Channel management page (ChannelsView.vue, 1065 lines): table list, search, status toggle, create/edit dialogs
• Pricing entry card (PricingEntryCard.vue): Token/per-request/image mode switching with interval config
• Model tag input (ModelTagInput.vue): Enter to add, paste for batch import
• Interval row (IntervalRow.vue): Context interval pricing configuration
• Sidebar navigation: New channel management entry
• Route registration: /admin/channels route
• Full i18n: English and Chinese translations (102 lines/language)

测试 / Tests

• Channel service unit tests (2187 lines)
• Channel handler unit tests (502 lines)
• Channel model unit tests (435 lines)
• Channel repository unit tests (227 lines)
• Account handler mixed-channel test