feat: 算力引擎 Go 后端核心（new-api）

- 统一 OpenAI 兼容 /v1 中继 + 渠道/额度/令牌/流水
- 各领域包：relay 模型网关、model 数据层、controller 管理 API

relay/channel/xai/constants.go

@@ -0,0 +1,32 @@
+package xai
+
+var ModelList = []string{
+	// language models
+	"grok-4-1-fast-reasoning",
+	"grok-4-1-fast-non-reasoning",
+	"grok-code-fast-1",
+	"grok-4-fast-reasoning",
+	"grok-4-fast-non-reasoning",
+	"grok-4-0709",
+	"grok-3-mini",
+	"grok-3",
+	"grok-2-vision-1212",
+	// search variants
+	"grok-4-1-fast-reasoning-search",
+	"grok-4-1-fast-non-reasoning-search",
+	"grok-4-fast-reasoning-search",
+	"grok-4-fast-non-reasoning-search",
+	"grok-4-0709-search",
+	"grok-3-mini-search",
+	"grok-3-search",
+	// grok-3-mini reasoning effort variants
+	"grok-3-mini-high", "grok-3-mini-low",
+	// image generation models
+	"grok-imagine-image-pro",
+	"grok-imagine-image",
+	"grok-2-image-1212",
+	// video generation model
+	"grok-imagine-video",
+}
+
+var ChannelName = "xai"