fix(db): 缩短 license_key 列至 VARCHAR(768) 避免 InnoDB 索引超限 [tsk_Qmk4wT_dkqRR]

原 size:2048 在 utf8mb4 下产生 8192 字节索引，超过 InnoDB 3072 字节上限，
全新环境 AutoMigrate 必失败。改为 VARCHAR(768)（768×4=3072 字节，恰好
在上限内），schema.sql 同步更新列类型并将 uk_license_key 改为全列索引（无
需前缀）。新增测试 TestLicenseTokenFitsColumnLimit 验证最坏情况 token 长度
确实在 768 字符以内。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/util/license_key_test.go

@@ -73,3 +73,38 @@ func TestVerifyLicenseToken_InvalidFormat(t *testing.T) {
 	_, err = VerifyLicenseToken("not-a-valid-token", pub)
 	assert.ErrorIs(t, err, ErrInvalidLicenseToken)
 }
+
+// TestLicenseTokenFitsColumnLimit verifies that a realistic (even worst-case)
+// license token fits within the VARCHAR(768) column limit imposed by the
+// InnoDB index constraint (768 chars × 4 bytes/char = 3072 bytes max).
+func TestLicenseTokenFitsColumnLimit(t *testing.T) {
+	const columnLimit = 768
+
+	priv, _, err := GenerateEd25519KeyPair()
+	require.NoError(t, err)
+
+	// Use a large features map to simulate a worst-case payload.
+	exp := time.Now().Add(365 * 24 * time.Hour).Unix()
+	payload := LicensePayload{
+		ShopID:    999999999,
+		LicenseID: 999999999,
+		Type:      "lifetime",
+		IssuedAt:  time.Now().Unix(),
+		ExpiresAt: &exp,
+		MaxDevices: 99,
+		Features: map[string]any{
+			"finance":    true,
+			"inventory":  true,
+			"reports":    true,
+			"export":     true,
+			"multi_shop": true,
+			"api_access": true,
+		},
+	}
+
+	token, err := IssueLicenseToken(payload, priv)
+	require.NoError(t, err)
+
+	assert.LessOrEqual(t, len(token), columnLimit,
+		"license token length %d exceeds VARCHAR(%d) column limit", len(token), columnLimit)
+}