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>

openspec/specs/task-management/spec.md

@@ -277,6 +277,53 @@ The system SHALL allow assigning tasks to users during creation and editing.
   - resource_id: 被刪除的專案 ID
   - changes: [{ field: "is_active", old_value: true, new_value: false }]
 
+### Requirement: Task Dependency Cycle Prevention
+The system SHALL detect and prevent circular dependencies between tasks to ensure Gantt charts can be properly rendered.
+
+#### Scenario: Direct circular dependency rejected
+- **WHEN** user attempts to create dependency where Task A depends on Task B and Task B depends on Task A
+- **THEN** system rejects the operation with 400 Bad Request
+- **THEN** error message includes the cycle path (e.g., "Circular dependency detected: A -> B -> A")
+
+#### Scenario: Indirect circular dependency rejected
+- **WHEN** user attempts to create dependency that would form a cycle (A -> B -> C -> A)
+- **THEN** system rejects the operation with 400 Bad Request
+- **THEN** error message includes the full cycle path
+
+#### Scenario: Valid dependency chain accepted
+- **WHEN** user creates dependencies forming a valid DAG (directed acyclic graph)
+- **THEN** system accepts and saves the dependencies
+- **THEN** Gantt chart renders correctly with proper task ordering
+
+### Requirement: Optimistic Locking for Task Updates
+The system SHALL use optimistic locking to prevent concurrent update conflicts on tasks.
+
+#### Scenario: Concurrent update detected
+- **WHEN** user A and user B both load task at version 1
+- **WHEN** user A saves changes, incrementing version to 2
+- **WHEN** user B attempts to save with version 1
+- **THEN** system returns 409 Conflict error
+- **THEN** error message instructs user to refresh and retry
+
+#### Scenario: Sequential updates succeed
+- **WHEN** user loads task at version N
+- **WHEN** user saves changes with correct version N
+- **THEN** system accepts update and increments version to N+1
+
+### Requirement: Soft Delete Cascade Restore
+The system SHALL restore child tasks when parent task is restored from soft delete.
+
+#### Scenario: Parent task restored with children
+- **WHEN** soft-deleted parent task is restored
+- **THEN** system identifies child tasks deleted at same timestamp
+- **THEN** system recursively restores all matching child tasks
+- **THEN** audit log records restoration of parent and children
+
+#### Scenario: Selective restore without children
+- **WHEN** user explicitly requests restore without cascade
+- **THEN** only parent task is restored
+- **THEN** child tasks remain in deleted state
+
 ## Data Model
 
 ```