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/api/comments/router.py

@@ -1,9 +1,11 @@
 import uuid
 from typing import List
-from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi import APIRouter, Depends, HTTPException, status, Request
 from sqlalchemy.orm import Session
 
 from app.core.database import get_db
+from app.core.rate_limiter import limiter
+from app.core.config import settings
 from app.models import User, Task, Comment
 from app.schemas.comment import (
     CommentCreate, CommentUpdate, CommentResponse, CommentListResponse,
@@ -49,13 +51,19 @@ def comment_to_response(comment: Comment) -> CommentResponse:
 
 
 @router.post("/api/tasks/{task_id}/comments", response_model=CommentResponse, status_code=status.HTTP_201_CREATED)
+@limiter.limit(settings.RATE_LIMIT_STANDARD)
 async def create_comment(
+    request: Request,
     task_id: str,
     comment_data: CommentCreate,
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
-    """Create a new comment on a task."""
+    """
+    Create a new comment on a task.
+
+    Rate limited: 60 requests per minute (standard tier).
+    """
     task = db.query(Task).filter(Task.id == task_id).first()
 
     if not task: