BE-PR-003: Add Essential SQLAlchemy model

- New app/models/essential.py with Essential model and EssentialType enum
  (feature, improvement, refactor)
- Fields: id, essential_code (unique), proposal_id (FK to proposes),
  type, title, description, created_by_id (FK to users), created_at, updated_at
- Added essentials relationship to Proposal model (cascade delete-orphan)
- Added essentials table auto-migration in main.py _migrate_schema()
- Registered essential module import in startup()

plans/TASKLIST.md

@@ -40,11 +40,11 @@
   - [x] 确认 Proposal 与 `project_id / created_by_id / feat_task_id` 的后续关系
   - [x] 标记 `feat_task_id` 是否废弃或替换
 
-- [ ] BE-PR-003:新增 Essential 模型(backend only)
-  - [ ] 新增 `Essential` SQLAlchemy model
-  - [ ] 增加 `proposal_id` 外键
-  - [ ] 增加 `essential_code`
-  - [ ] 增加 `type / title / description / created_by_id / created_at / updated_at`
+- [x] BE-PR-003:新增 Essential 模型(backend only)
+  - [x] 新增 `Essential` SQLAlchemy model
+  - [x] 增加 `proposal_id` 外键
+  - [x] 增加 `essential_code`
+  - [x] 增加 `type / title / description / created_by_id / created_at / updated_at`
 
 - [ ] BE-PR-004:实现 EssentialCode 编码规则(backend only)
   - [ ] 参考现有 `ProjectCode / MilestoneCode / TaskCode / ProposeCode` 生成方式