feat: implement custom fields, gantt view, calendar view, and file encryption

- Custom Fields (FEAT-001):
  - CustomField and TaskCustomValue models with formula support
  - CRUD API for custom field management
  - Formula engine for calculated fields
  - Frontend: CustomFieldEditor, CustomFieldInput, ProjectSettings page
  - Task list API now includes custom_values
  - KanbanBoard displays custom field values

- Gantt View (FEAT-003):
  - TaskDependency model with FS/SS/FF/SF dependency types
  - Dependency CRUD API with cycle detection
  - start_date field added to tasks
  - GanttChart component with Frappe Gantt integration
  - Dependency type selector in UI

- Calendar View (FEAT-004):
  - CalendarView component with FullCalendar integration
  - Date range filtering API for tasks
  - Drag-and-drop date updates
  - View mode switching in Tasks page

- File Encryption (FEAT-010):
  - AES-256-GCM encryption service
  - EncryptionKey model with key rotation support
  - Admin API for key management
  - Encrypted upload/download for confidential projects

- Migrations: 011 (custom fields), 012 (encryption keys), 013 (task dependencies)
- Updated issues.md with completion status

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

openspec/changes/archive/2026-01-05-add-gantt-view/tasks.md

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+# Tasks: Add Gantt View
+
+## Backend Tasks
+
+### 1. Add start_date to Task model
+- [x] Add `start_date` column to Task model
+- [x] Create Alembic migration
+- [x] Update TaskCreate/TaskUpdate schemas to include start_date
+- [x] Update TaskResponse schema
+- **驗證**: Migration 成功，API 可設定 start_date
+
+### 2. Create TaskDependency model
+- [x] Create `backend/app/models/task_dependency.py`
+- [x] Define predecessor_id, successor_id, dependency_type, lag_days
+- [x] Update `backend/app/models/__init__.py`
+- [x] Create Alembic migration
+- **驗證**: Migration 成功
+
+### 3. Implement dependency CRUD API
+- [x] Create `backend/app/api/task_dependencies/router.py`
+- [x] Implement `POST /api/tasks/{task_id}/dependencies` - 新增依賴
+- [x] Implement `GET /api/tasks/{task_id}/dependencies` - 取得依賴
+- [x] Implement `DELETE /api/task-dependencies/{dependency_id}` - 刪除依賴
+- [x] Add circular dependency detection
+- [x] Register router in main.py
+- **驗證**: 依賴關係 CRUD 可正常操作，循環依賴被拒絕
+
+### 4. Add date validation
+- [x] Validate start_date <= due_date
+- [x] Validate dependency constraints on date change
+- [x] Add validation tests
+- **驗證**: 不合理日期被拒絕
+
+### 5. Add backend tests
+- [x] Test TaskDependency CRUD
+- [x] Test circular dependency detection
+- [x] Test date validation
+- **驗證**: 所有測試通過
+
+## Frontend Tasks
+
+### 6. Install and configure Gantt library
+- [x] Install Frappe Gantt (or chosen library)
+- [x] Create wrapper component for React integration
+- [x] Configure styling to match application theme
+- **驗證**: Gantt 元件可正常渲染
+
+### 7. Create GanttChart component
+- [x] Create `frontend/src/components/GanttChart.tsx`
+- [x] Load tasks with start_date and due_date
+- [x] Display tasks as horizontal bars on timeline
+- [x] Show task title, assignee, progress
+- [x] Support zoom levels (day, week, month)
+- **驗證**: 任務正確顯示在時間軸上
+
+### 8. Implement drag-to-edit dates
+- [x] Handle bar drag to move task dates
+- [x] Handle bar resize to change duration
+- [x] Call API to update task dates
+- [x] Show optimistic update, rollback on error
+- **驗證**: 拖拉調整日期可正確更新
+
+### 9. Implement dependency visualization
+- [x] Add dependency arrows between tasks
+- [x] Create dependency API service
+- [x] Implement add/remove dependency UI (right-click or toolbar)
+- [x] Validate and show error for circular dependencies
+- **驗證**: 依賴關係正確顯示和編輯
+
+### 10. Integrate Gantt view into Tasks page
+- [x] Add "Gantt" option to view toggle
+- [x] Store view preference in localStorage
+- [x] Handle view switching
+- **驗證**: 可在 List/Kanban/Gantt 之間切換
+
+## Task Dependencies
+
+```
+[1] start_date 欄位
+      ↓
+[2] TaskDependency Model → [3] Dependency API → [4] Date Validation → [5] Tests
+                                    ↓
+[6] Gantt Library Setup → [7] GanttChart Component
+                                    ↓
+                          [8] Drag Edit → [9] Dependency UI
+                                    ↓
+                          [10] View Integration
+```
+
+- Tasks 1-5 (Backend) 可平行於 Tasks 6-10 (Frontend) 開發
+- Task 7 需要 Task 1 完成（需要 start_date 欄位）
+- Task 9 需要 Task 3 完成（需要依賴 API）