feat: implement 8 OpenSpec proposals for security, reliability, and UX improvements

## Security Enhancements (P0)
- Add input validation with max_length and numeric range constraints
- Implement WebSocket token authentication via first message
- Add path traversal prevention in file storage service

## Permission Enhancements (P0)
- Add project member management for cross-department access
- Implement is_department_manager flag for workload visibility

## Cycle Detection (P0)
- Add DFS-based cycle detection for task dependencies
- Add formula field circular reference detection
- Display user-friendly cycle path visualization

## Concurrency & Reliability (P1)
- Implement optimistic locking with version field (409 Conflict on mismatch)
- Add trigger retry mechanism with exponential backoff (1s, 2s, 4s)
- Implement cascade restore for soft-deleted tasks

## Rate Limiting (P1)
- Add tiered rate limits: standard (60/min), sensitive (20/min), heavy (5/min)
- Apply rate limits to tasks, reports, attachments, and comments

## Frontend Improvements (P1)
- Add responsive sidebar with hamburger menu for mobile
- Improve touch-friendly UI with proper tap target sizes
- Complete i18n translations for all components

## Backend Reliability (P2)
- Configure database connection pool (size=10, overflow=20)
- Add Redis fallback mechanism with message queue
- Add blocker check before task deletion

## API Enhancements (P3)
- Add standardized response wrapper utility
- Add /health/ready and /health/live endpoints
- Implement project templates with status/field copying

## Tests Added
- test_input_validation.py - Schema and path traversal tests
- test_concurrency_reliability.py - Optimistic locking and retry tests
- test_backend_reliability.py - Connection pool and Redis tests
- test_api_enhancements.py - Health check and template tests

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

frontend/public/locales/zh-TW/tasks.json

@@ -181,5 +181,33 @@
     "title": "沒有任務",
     "description": "目前沒有任務。建立您的第一個任務開始吧！",
     "filtered": "沒有符合篩選條件的任務"
+  },
+  "dependencies": {
+    "title": "任務相依性",
+    "add": "新增相依性",
+    "remove": "移除相依性",
+    "predecessor": "前置任務",
+    "successor": "後續任務",
+    "type": "相依性類型",
+    "types": {
+      "FS": "完成後開始",
+      "SS": "同時開始",
+      "FF": "同時完成",
+      "SF": "開始後完成"
+    },
+    "circularError": {
+      "title": "偵測到循環相依",
+      "description": "新增此相依性會產生循環參照，這是不被允許的。",
+      "cyclePath": "相依性循環路徑",
+      "helpText": "要解決此問題，請選擇不同的任務作為相依項，或移除循環中現有的某個相依關係。"
+    },
+    "error": {
+      "addFailed": "新增相依性失敗",
+      "removeFailed": "移除相依性失敗"
+    }
+  },
+  "conflict": {
+    "title": "更新衝突",
+    "message": "此任務已被其他使用者修改。請重新整理頁面以取得最新版本，然後再試一次。"
   }
 }