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/proxy/handlers.go

@@ -0,0 +1,49 @@
+package proxy
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// HandleChat handles POST /v1/chat/completions
+func (g *Gateway) HandleChat(c *gin.Context) {
+	req, err := g.ParseRequest(c, "chat")
+	if err != nil {
+		g.writeError(c, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	g.Dispatch(c, req)
+}
+
+// HandleMessages handles POST /v1/messages
+func (g *Gateway) HandleMessages(c *gin.Context) {
+	req, err := g.ParseRequest(c, "messages")
+	if err != nil {
+		g.writeError(c, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	g.Dispatch(c, req)
+}
+
+// HandleResponses handles POST /v1/responses
+func (g *Gateway) HandleResponses(c *gin.Context) {
+	req, err := g.ParseRequest(c, "responses")
+	if err != nil {
+		g.writeError(c, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	g.Dispatch(c, req)
+}
+
+// HandleModels handles GET /v1/models
+func (g *Gateway) HandleModels(c *gin.Context) {
+	// TODO: Return list of available models based on enabled channels
+	c.JSON(http.StatusOK, gin.H{
+		"object": "list",
+		"data":   []interface{}{},
+	})
+}