feat: Implement public access for tenant content

- Add TenantOptionalAuth middleware to allow access to public content without requiring authentication.
- Introduce ListPublicPublished and PublicDetail methods in the content service to retrieve publicly accessible content.
- Create tenant_public HTTP routes for listing and showing public content, including preview and main asset retrieval.
- Enhance content tests to cover scenarios for public content access and permissions.
- Update specifications to reflect the new public content access features and rules.

backend/app/http/tenant_join/dto/join.go

@@ -0,0 +1,13 @@
+package dto
+
+// JoinByInviteForm 用户通过邀请码加入租户的请求参数。
+type JoinByInviteForm struct {
+	// InviteCode 邀请码：由租户管理员生成；用户提交后加入对应租户。
+	InviteCode string `json:"invite_code"`
+}
+
+// JoinRequestCreateForm 用户提交加入租户申请的请求参数（无邀请码场景）。
+type JoinRequestCreateForm struct {
+	// Reason 申请原因（可选）：用于向租户管理员说明申请加入的目的。
+	Reason string `json:"reason"`
+}