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>

backend/app/models/__init__.py

@@ -23,6 +23,8 @@ from app.models.project_health import ProjectHealth, RiskLevel, ScheduleStatus,
 from app.models.custom_field import CustomField, FieldType
 from app.models.task_custom_value import TaskCustomValue
 from app.models.task_dependency import TaskDependency, DependencyType
+from app.models.project_member import ProjectMember
+from app.models.project_template import ProjectTemplate
 
 __all__ = [
     "User", "Role", "Department", "Space", "Project", "TaskStatus", "Task", "WorkloadSnapshot",
@@ -33,5 +35,7 @@ __all__ = [
     "ScheduledReport", "ReportType", "ReportHistory", "ReportHistoryStatus",
     "ProjectHealth", "RiskLevel", "ScheduleStatus", "ResourceStatus",
     "CustomField", "FieldType", "TaskCustomValue",
-    "TaskDependency", "DependencyType"
+    "TaskDependency", "DependencyType",
+    "ProjectMember",
+    "ProjectTemplate"
 ]