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/attachments/router.py

@@ -8,6 +8,8 @@ from sqlalchemy.orm import Session
 from typing import Optional
 
 from app.core.database import get_db
+from app.core.rate_limiter import limiter
+from app.core.config import settings
 from app.middleware.auth import get_current_user, check_task_access, check_task_edit_access
 from app.models import User, Task, Project, Attachment, AttachmentVersion, EncryptionKey, AuditAction
 from app.schemas.attachment import (
@@ -156,9 +158,10 @@ def should_encrypt_file(project: Project, db: Session) -> tuple[bool, Optional[E
 
 
 @router.post("/tasks/{task_id}/attachments", response_model=AttachmentResponse)
+@limiter.limit(settings.RATE_LIMIT_SENSITIVE)
 async def upload_attachment(
-    task_id: str,
     request: Request,
+    task_id: str,
     file: UploadFile = File(...),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user)
@@ -167,6 +170,8 @@ async def upload_attachment(
     Upload a file attachment to a task.
 
     For confidential projects, files are automatically encrypted using AES-256-GCM.
+
+    Rate limited: 20 requests per minute (sensitive tier).
     """
     task = get_task_with_access_check(db, task_id, current_user, require_edit=True)