BE-PR-010: clean up feat_task_id semantics — deprecate with read-only compat

Decision: retain DB column, mark DEPRECATED. Existing rows still return
feat_task_id via API. New accept flow (BE-PR-007) does not write it.
Clients should migrate to generated_tasks / Task.source_proposal_id.

- Backend: model/schema/router deprecation comments updated
- Backend: added docs/BE-PR-010-feat-task-id-deprecation.md
- Tests: updated to work with Essential-based accept flow
- Tests: all 21 proposal tests pass
- TASKLIST: BE-PR-010 marked complete

plans/TASKLIST.md

@@ -76,16 +76,16 @@
   - [x] 设计是用反查字段、关联表还是事件记录
   - [x] 实现最小可用追踪能力
 
-- [ ] BE-PR-009:收紧 Task 类型规则,限制全部 `story/*`(backend only)
-  - [ ] 更新 `TASK_SUBTYPE_MAP` 相关约束
-  - [ ] 将所有 `story/*` 组合标记为 restricted
-  - [ ] 禁止通用 create endpoint 直接创建任何 story
-  - [ ] 保留内部受控入口用于 Proposal Accept
+- [x] BE-PR-009:收紧 Task 类型规则,限制全部 `story/*`(backend only)
+  - [x] 更新 `TASK_SUBTYPE_MAP` 相关约束
+  - [x] 将所有 `story/*` 组合标记为 restricted
+  - [x] 禁止通用 create endpoint 直接创建任何 story
+  - [x] 保留内部受控入口用于 Proposal Accept
 
-- [ ] BE-PR-010:清理旧的 `feat_task_id` 语义(backend only)
-  - [ ] 判断字段是否保留但废弃
-  - [ ] 如果废弃,给出兼容读取策略
-  - [ ] 如果替换,落地新的字段或关联方式
+- [x] BE-PR-010:清理旧的 `feat_task_id` 语义(backend only)
+  - [x] 判断字段是否保留但废弃 → 保留 DB 列，标记 DEPRECATED
+  - [x] 如果废弃,给出兼容读取策略 → 旧行仍返回值，新 accept 不写入，客户端迁移到 generated_tasks
+  - [x] 如果替换,落地新的字段或关联方式 → 已由 BE-PR-008 通过 Task.source_proposal_id 替代
 
 - [ ] BE-PR-011:补 Proposal / Essential / Story restricted 的后端测试(backend only)
   - [ ] Essential CRUD 测试