docs(workflow): clarify version vs version_field naming in process_definition

Add doc comments to distinguish the business version field (key-based
revision counter, currently fixed at 1) from the optimistic lock field
(version_field). Renaming requires a DB migration so deferred to later.

crates/erp-workflow/src/entity/process_definition.rs

@@ -9,6 +9,7 @@ pub struct Model {
     pub tenant_id: Uuid,
     pub name: String,
     pub key: String,
+    /// 业务版本号（如同一 key 可存在多个版本），当前固定为 1，未来支持发布新版本时递增。
     pub version: i32,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub category: Option<String>,
@@ -23,6 +24,7 @@ pub struct Model {
     pub updated_by: Uuid,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub deleted_at: Option<DateTimeUtc>,
+    /// 乐观锁版本号（ERP 标准审计字段），每次更新递增。
     pub version_field: i32,
 }