refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/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)
+}