feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/internal/pkg/httpx/httpx.go

@@ -0,0 +1,42 @@
+package httpx
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// APIError is the standard error body for the management API.
+type APIError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+}
+
+// OK writes a JSON success response.
+func OK(c *gin.Context, data any) {
+	c.JSON(http.StatusOK, gin.H{"code": 0, "data": data})
+}
+
+// Created writes a 201 response.
+func Created(c *gin.Context, data any) {
+	c.JSON(http.StatusCreated, gin.H{"code": 0, "data": data})
+}
+
+// Fail writes an error response with the given status.
+func Fail(c *gin.Context, status int, message string) {
+	c.AbortWithStatusJSON(status, gin.H{"code": status, "message": message})
+}
+
+// FailWithCode writes an error with a custom business code.
+func FailWithCode(c *gin.Context, status, code int, message string) {
+	c.AbortWithStatusJSON(status, gin.H{"code": code, "message": message})
+}
+
+// Bind parses the JSON body and aborts with 400 on failure.
+func Bind(c *gin.Context, dst any) bool {
+	if err := c.ShouldBindJSON(dst); err != nil {
+		Fail(c, http.StatusBadRequest, "invalid request body: "+err.Error())
+		return false
+	}
+	return true
+}