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/changes/archive/2026-01-10-add-concurrency-reliability/tasks.md

@@ -0,0 +1,29 @@
+# Tasks: Add Concurrency Handling and Reliability Improvements
+
+## 1. Optimistic Locking
+- [x] 1.1 Add `version` integer field to Task model with default=1
+- [x] 1.2 Create database migration for version field
+- [x] 1.3 Include version in TaskResponse schema
+- [x] 1.4 Modify update_task to accept and validate version
+- [x] 1.5 Return 409 Conflict when version mismatch detected
+- [x] 1.6 Auto-increment version on successful update
+- [x] 1.7 Update frontend to send version with update requests
+- [x] 1.8 Handle 409 Conflict in frontend with user notification
+
+## 2. Trigger Retry Mechanism
+- [x] 2.1 Add retry configuration (max_retries=3, base_delay=1s)
+- [x] 2.2 Implement exponential backoff (1s, 2s, 4s)
+- [x] 2.3 Log each retry attempt with attempt number
+- [x] 2.4 Mark trigger as permanently failed after max retries
+- [x] 2.5 Send alert notification when trigger exhausts retries
+
+## 3. Soft Delete Cascade Restore
+- [x] 3.1 Modify restore_task to find all child tasks deleted at same time
+- [x] 3.2 Recursively restore child tasks with matching deleted_at timestamp
+- [x] 3.3 Add option to restore only parent vs cascade restore
+- [x] 3.4 Log restore operations in audit trail
+
+## 4. Testing
+- [x] 4.1 Test concurrent updates with version conflict
+- [x] 4.2 Test trigger retry on transient failure
+- [x] 4.3 Test cascade restore of parent with children