feat(attach): 附件可配置限额、流式落盘与分阶段上传

管理端可配类型/体积；上传改流式避免整文件进内存；发帖先选再传并拦截未上传提交；提高 Next proxy 体积上限。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/service/attachment_settings_test.go

@@ -0,0 +1,36 @@
+package service
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestAttachmentExtAllowedLogic(t *testing.T) {
+	// Normalize + default list contains msi/html
+	found := false
+	for _, e := range DefaultAttachmentExts {
+		if e == "msi" {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatal("default list should include msi")
+	}
+}
+
+func TestMimeForAttachmentExt(t *testing.T) {
+	if MimeForAttachmentExt(".pdf") == "application/octet-stream" {
+		t.Fatal("pdf should have real mime")
+	}
+	if MimeForAttachmentExt(".html") != "application/octet-stream" {
+		t.Fatal("html should be octet-stream")
+	}
+}
+
+func TestFileMaxBytesDefault(t *testing.T) {
+	if FileMaxBytes != 20<<20 {
+		t.Fatalf("default FileMaxBytes=%d", FileMaxBytes)
+	}
+	_ = fmt.Sprintf("%d", DefaultAttachmentMaxMB)
+}