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/schemas/project_member.py

@@ -0,0 +1,56 @@
+"""Project member schemas for cross-department collaboration."""
+from pydantic import BaseModel, Field
+from typing import Optional, List
+from datetime import datetime
+from enum import Enum
+
+
+class ProjectMemberRole(str, Enum):
+    """Roles that can be assigned to project members."""
+    MEMBER = "member"
+    ADMIN = "admin"
+
+
+class ProjectMemberBase(BaseModel):
+    """Base schema for project member."""
+    user_id: str = Field(..., description="ID of the user to add as project member")
+    role: ProjectMemberRole = Field(
+        default=ProjectMemberRole.MEMBER,
+        description="Role of the member: 'member' (view/edit tasks) or 'admin' (manage project)"
+    )
+
+
+class ProjectMemberCreate(ProjectMemberBase):
+    """Schema for creating a project member."""
+    pass
+
+
+class ProjectMemberUpdate(BaseModel):
+    """Schema for updating a project member."""
+    role: ProjectMemberRole = Field(..., description="New role for the member")
+
+
+class ProjectMemberResponse(ProjectMemberBase):
+    """Schema for project member response."""
+    id: str
+    project_id: str
+    added_by: str
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class ProjectMemberWithDetails(ProjectMemberResponse):
+    """Schema for project member with user details."""
+    user_name: Optional[str] = None
+    user_email: Optional[str] = None
+    user_department_id: Optional[str] = None
+    user_department_name: Optional[str] = None
+    added_by_name: Optional[str] = None
+
+
+class ProjectMemberListResponse(BaseModel):
+    """Schema for listing project members."""
+    members: List[ProjectMemberWithDetails]
+    total: int