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_template.py

@@ -0,0 +1,95 @@
+"""Schemas for project template API endpoints."""
+from typing import Optional, List, Any
+from datetime import datetime
+from pydantic import BaseModel, Field
+
+
+class TaskStatusDefinition(BaseModel):
+    """Task status definition for templates."""
+    name: str = Field(..., min_length=1, max_length=50)
+    color: str = Field(default="#808080", pattern=r"^#[0-9A-Fa-f]{6}$")
+    position: int = Field(default=0, ge=0)
+    is_done: bool = Field(default=False)
+
+
+class CustomFieldDefinition(BaseModel):
+    """Custom field definition for templates."""
+    name: str = Field(..., min_length=1, max_length=100)
+    field_type: str = Field(..., pattern=r"^(text|number|dropdown|date|person|formula)$")
+    options: Optional[List[str]] = None
+    formula: Optional[str] = None
+    is_required: bool = Field(default=False)
+    position: int = Field(default=0, ge=0)
+
+
+class ProjectTemplateBase(BaseModel):
+    """Base schema for project template."""
+    name: str = Field(..., min_length=1, max_length=200)
+    description: Optional[str] = None
+    is_public: bool = Field(default=False)
+    task_statuses: Optional[List[TaskStatusDefinition]] = None
+    custom_fields: Optional[List[CustomFieldDefinition]] = None
+    default_security_level: Optional[str] = Field(
+        default="department",
+        pattern=r"^(public|department|confidential)$"
+    )
+
+
+class ProjectTemplateCreate(ProjectTemplateBase):
+    """Schema for creating a project template."""
+    pass
+
+
+class ProjectTemplateUpdate(BaseModel):
+    """Schema for updating a project template."""
+    name: Optional[str] = Field(None, min_length=1, max_length=200)
+    description: Optional[str] = None
+    is_public: Optional[bool] = None
+    task_statuses: Optional[List[TaskStatusDefinition]] = None
+    custom_fields: Optional[List[CustomFieldDefinition]] = None
+    default_security_level: Optional[str] = Field(
+        None,
+        pattern=r"^(public|department|confidential)$"
+    )
+
+
+class ProjectTemplateResponse(ProjectTemplateBase):
+    """Schema for project template response."""
+    id: str
+    owner_id: str
+    is_active: bool
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class ProjectTemplateWithOwner(ProjectTemplateResponse):
+    """Project template response with owner details."""
+    owner_name: Optional[str] = None
+
+
+class ProjectTemplateListResponse(BaseModel):
+    """Response schema for listing project templates."""
+    templates: List[ProjectTemplateWithOwner]
+    total: int
+
+
+class CreateProjectFromTemplateRequest(BaseModel):
+    """Request schema for creating a project from a template."""
+    template_id: str
+    title: str = Field(..., min_length=1, max_length=500)
+    description: Optional[str] = Field(None, max_length=10000)
+    space_id: str
+    department_id: Optional[str] = None
+
+
+class CreateProjectFromTemplateResponse(BaseModel):
+    """Response schema for project created from template."""
+    id: str
+    title: str
+    template_id: str
+    template_name: str
+    task_statuses_created: int
+    custom_fields_created: int