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-cycle-detection/specs/task-management/spec.md

@@ -0,0 +1,19 @@
+## ADDED Requirements
+
+### 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