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/apikey/apikey.go

@@ -0,0 +1,30 @@
+package apikey
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"opencatd-open/internal/pkg/crypto"
+	"strings"
+)
+
+const Prefix = "sk-ot-"
+
+// Generate 生成新的 API Key，返回明文和哈希
+func Generate() (plaintext, hash string) {
+	b := make([]byte, 24)
+	_, _ = rand.Read(b)
+	raw := hex.EncodeToString(b)
+	plaintext = Prefix + raw
+	hash = crypto.Sha256Hex(plaintext)
+	return
+}
+
+// Valid 校验 API Key 格式
+func Valid(key string) bool {
+	return strings.HasPrefix(key, Prefix)
+}
+
+// Hash 计算 API Key 的 SHA-256 哈希
+func Hash(key string) string {
+	return crypto.Sha256Hex(key)
+}