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/admin/config.go

@@ -0,0 +1,57 @@
+package admin
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
+	"gorm.io/gorm"
+
+	"openteam/server/internal/pkg/httpx"
+	"openteam/server/internal/store"
+)
+
+type configHandler struct {
+	db  *gorm.DB
+	log *zap.Logger
+}
+
+// Get handles GET /api/v1/admin/config — returns the full config map.
+func (h *configHandler) Get(c *gin.Context) {
+	var confs []store.SystemConfig
+	if err := h.db.Find(&confs).Error; err != nil {
+		h.log.Warn("load config failed", zap.Error(err))
+		httpx.Fail(c, http.StatusInternalServerError, "load config failed")
+		return
+	}
+	out := map[string]any{}
+	for _, conf := range confs {
+		var v any
+		if err := json.Unmarshal(conf.Value, &v); err == nil {
+			out[conf.Key] = v
+		}
+	}
+	httpx.OK(c, out)
+}
+
+// Put handles PUT /api/v1/admin/config — upserts key/value pairs.
+func (h *configHandler) Put(c *gin.Context) {
+	var in map[string]any
+	if !httpx.Bind(c, &in) {
+		return
+	}
+	for k, v := range in {
+		raw, err := json.Marshal(v)
+		if err != nil {
+			continue
+		}
+		conf := store.SystemConfig{Key: k, Value: raw}
+		if err := h.db.Save(&conf).Error; err != nil {
+			h.log.Warn("save config failed", zap.Error(err), zap.String("key", k))
+			httpx.Fail(c, http.StatusInternalServerError, "save config failed")
+			return
+		}
+	}
+	httpx.OK(c, gin.H{"ok": true})
+}