refactor: complete backend rewrite for multi-protocol proxy

Major rewrite of the Go backend to support:
- Three API format imports: openai, anthropic, compatible
- Three protocol conversions: Chat Completions, Responses, Messages
- Hub-and-spoke architecture with Chat as intermediate format

Deleted:
- opencat.go (old entry)
- store/, team/, pkg/team/, pkg/store/ (old data layer)
- internal/model/, internal/consts/ (old types)
- internal/service/team/, internal/controller/team/ (old handlers)
- llm/ (removed LLM client library, pure proxy mode)
- dist/, assets/ (old build artifacts)

Added:
- internal/store/ — 9 GORM models + multi-DB support
- internal/pkg/ — crypto (AES-GCM), apikey, jwt, ratelimit, resp, tokenizer
- internal/channel/ — channel selection, weighted LB, health checks
- internal/proxy/convert/ — 6 protocol conversion functions + SSE streaming
- internal/proxy/ — gateway with request dispatch and upstream selection
- internal/usage/ — async usage recorder with batch writes
- internal/api/ — management API (auth, users, keys, channels, models)
- Makefile for build/test/deploy

Fixed API to match frontend expectations:
- Login response wraps token in { data: { token } }
- GET /api/profile route added
- Profile response wraps user in { code, data }
- Role returned as number (10=admin, 1=user)

internal/pkg/resp/resp.go

@@ -0,0 +1,47 @@
+package resp
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// Error 按 OpenAI 格式返回错误
+func Error(c *gin.Context, status int, message string) {
+	c.AbortWithStatusJSON(status, gin.H{
+		"error": gin.H{
+			"message": message,
+			"type":    "api_error",
+			"param":   nil,
+			"code":    nil,
+		},
+	})
+}
+
+// ErrorWithType 按 OpenAI 格式返回带类型的错误
+func ErrorWithType(c *gin.Context, status int, errType, code, message string) {
+	c.AbortWithStatusJSON(status, gin.H{
+		"error": gin.H{
+			"message": message,
+			"type":    errType,
+			"param":   nil,
+			"code":    code,
+		},
+	})
+}
+
+// ErrorAsAnthropic 按 Anthropic 格式返回错误
+func ErrorAsAnthropic(c *gin.Context, status int, errType, message string) {
+	c.AbortWithStatusJSON(status, gin.H{
+		"type": "error",
+		"error": gin.H{
+			"type":    errType,
+			"message": message,
+		},
+	})
+}
+
+// OK 返回成功 JSON
+func OK(c *gin.Context, data interface{}) {
+	c.JSON(http.StatusOK, data)
+}