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/proposal.md

@@ -0,0 +1,17 @@
+# Change: Add Concurrency Handling and Reliability Improvements
+
+## Why
+Multiple users editing the same task simultaneously can lead to lost updates (last-write-wins). Trigger execution failures are not retried, causing permanent failures on transient errors. Soft-delete restore operation does not cascade to child tasks, leading to data inconsistency.
+
+## What Changes
+- Implement optimistic locking using version field for task updates
+- Add retry mechanism with exponential backoff for trigger execution
+- Implement cascade restore for soft-deleted tasks and their children
+
+## Impact
+- Affected specs: task-management, automation
+- Affected code:
+  - `backend/app/models/task.py` - Add version field
+  - `backend/app/api/tasks/router.py` - Optimistic locking logic
+  - `backend/app/services/trigger_scheduler.py` - Retry mechanism
+  - `backend/app/api/tasks/router.py` - Cascade restore