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-input-validation-security/proposal.md

@@ -0,0 +1,17 @@
+# Change: Add Input Validation and Security Enhancements
+
+## Why
+Current API endpoints lack comprehensive input validation, exposing the system to potential DoS attacks, database overflow errors, and security vulnerabilities. Additionally, WebSocket authentication tokens are exposed in query parameters which may be logged.
+
+## What Changes
+- Add length validation to all Pydantic schema string fields
+- Add numeric range validation for decimal/integer fields
+- Enhance WebSocket token handling to avoid query parameter exposure
+- Strengthen file path traversal protection in file storage service
+
+## Impact
+- Affected specs: user-auth
+- Affected code:
+  - `backend/app/schemas/*.py` - All schema files
+  - `backend/app/api/websocket/router.py` - WebSocket authentication
+  - `backend/app/services/file_storage_service.py` - Path validation