feat: implement 5 QA-driven security and quality proposals

Implemented proposals from comprehensive QA review:

1. extend-csrf-protection
   - Add POST to CSRF protected methods in frontend
   - Global CSRF middleware for all state-changing operations
   - Update tests with CSRF token fixtures

2. tighten-cors-websocket-security
   - Replace wildcard CORS with explicit method/header lists
   - Disable query parameter auth in production (code 4002)
   - Add per-user WebSocket connection limit (max 5, code 4005)

3. shorten-jwt-expiry
   - Reduce JWT expiry from 7 days to 60 minutes
   - Add refresh token support with 7-day expiry
   - Implement token rotation on refresh
   - Frontend auto-refresh when token near expiry (<5 min)

4. fix-frontend-quality
   - Add React.lazy() code splitting for all pages
   - Fix useCallback dependency arrays (Dashboard, Comments)
   - Add localStorage data validation in AuthContext
   - Complete i18n for AttachmentUpload component

5. enhance-backend-validation
   - Add SecurityAuditMiddleware for access denied logging
   - Add ErrorSanitizerMiddleware for production error messages
   - Protect /health/detailed with admin authentication
   - Add input length validation (comment 5000, desc 10000)

All 521 backend tests passing. Frontend builds successfully.

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

openspec/specs/task-management/spec.md

@@ -78,6 +78,26 @@
 - **WHEN** 管理者嘗試新增第 21 個欄位
 - **THEN** 系統拒絕新增並顯示數量已達上限的訊息
 
+### Requirement: Input Length Validation
+All text input fields SHALL have maximum length constraints to prevent abuse and database issues.
+
+#### Scenario: Task title exceeds limit
+- **WHEN** user creates a task with title exceeding 500 characters
+- **THEN** request is rejected with 422 Validation Error
+- **AND** error indicates field length exceeded
+
+#### Scenario: Description within limit
+- **WHEN** user creates a task with description 10000 characters or less
+- **THEN** task is created successfully
+
+#### Scenario: Description exceeds limit
+- **WHEN** user creates a task with description exceeding 10000 characters
+- **THEN** request is rejected with 422 Validation Error
+
+#### Scenario: Comment content limit
+- **WHEN** user submits a comment exceeding 5000 characters
+- **THEN** request is rejected with 422 Validation Error
+
 ### Requirement: Multiple Views
 系統 SHALL 支援多維視角：看板 (Kanban)、甘特圖 (Gantt)、列表 (List)、行事曆 (Calendar)。