feat: API Key 前缀改为 sk-ot

- apikey 包 prefix 常量 sk- → sk-ot（生成/校验/展示前缀统一驱动）
- 鉴权错误提示同步更新为 sk-ot 格式
- 测试断言改用常量，避免硬编码长度
- 前端 curl 示例与 README/PLANNING 文档同步

server/internal/pkg/apikey/apikey_test.go

@@ -3,32 +3,35 @@ package apikey
 import "testing"
 
 func TestGenerateValid(t *testing.T) {
-	plain, hash, prefix, err := Generate()
+	plain, hash, keyPrefix, err := Generate()
 	if err != nil {
 		t.Fatalf("Generate: %v", err)
 	}
 	if !Valid(plain) {
 		t.Fatalf("generated key invalid: %q", plain)
 	}
-	if len(plain) != len("sk-")+48 {
+	if len(plain) != len(prefix)+keyLen {
 		t.Fatalf("unexpected key length: %d", len(plain))
 	}
+	if keyPrefix != plain[:12] {
+		t.Fatalf("prefix mismatch: %s vs %s", keyPrefix, plain[:12])
+	}
 	if Hash(plain) != hash {
 		t.Fatal("hash mismatch")
 	}
-	if prefix != plain[:12] {
-		t.Fatalf("prefix mismatch: %s vs %s", prefix, plain[:12])
-	}
 }
 
 func TestHashStable(t *testing.T) {
-	if Hash("sk-test") != Hash("sk-test") {
+	if Hash("sk-ot-test") != Hash("sk-ot-test") {
 		t.Fatal("hash not stable")
 	}
 }
 
 func TestValidRejects(t *testing.T) {
-	cases := []string{"", "sk-abc", "abc-123456789012345678901234567890123456789012345678", "sk-1234567890123456789012345678901234567890123456789"}
+	tooShort := prefix + "abc"
+	tooLong := prefix + "1234567890123456789012345678901234567890123456789"
+	wrongPrefix := "abc-ot123456789012345678901234567890123456789012345678"
+	cases := []string{"", tooShort, wrongPrefix, tooLong}
 	for _, c := range cases {
 		if Valid(c) {
 			t.Fatalf("expected invalid: %q", c)