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/task.py

@@ -1,4 +1,4 @@
-from pydantic import BaseModel, computed_field
+from pydantic import BaseModel, computed_field, Field, field_validator
 from typing import Optional, List, Any, Dict
 from datetime import datetime
 from decimal import Decimal
@@ -28,10 +28,10 @@ class CustomValueResponse(BaseModel):
 
 
 class TaskBase(BaseModel):
-    title: str
-    description: Optional[str] = None
+    title: str = Field(..., min_length=1, max_length=500)
+    description: Optional[str] = Field(None, max_length=10000)
     priority: Priority = Priority.MEDIUM
-    original_estimate: Optional[Decimal] = None
+    original_estimate: Optional[Decimal] = Field(None, ge=0, le=99999)
     start_date: Optional[datetime] = None
     due_date: Optional[datetime] = None
 
@@ -44,17 +44,18 @@ class TaskCreate(TaskBase):
 
 
 class TaskUpdate(BaseModel):
-    title: Optional[str] = None
-    description: Optional[str] = None
+    title: Optional[str] = Field(None, min_length=1, max_length=500)
+    description: Optional[str] = Field(None, max_length=10000)
     priority: Optional[Priority] = None
     status_id: Optional[str] = None
     assignee_id: Optional[str] = None
-    original_estimate: Optional[Decimal] = None
-    time_spent: Optional[Decimal] = None
+    original_estimate: Optional[Decimal] = Field(None, ge=0, le=99999)
+    time_spent: Optional[Decimal] = Field(None, ge=0, le=99999)
     start_date: Optional[datetime] = None
     due_date: Optional[datetime] = None
-    position: Optional[int] = None
+    position: Optional[int] = Field(None, ge=0)
     custom_values: Optional[List[CustomValueInput]] = None
+    version: Optional[int] = Field(None, ge=1, description="Version for optimistic locking")
 
 
 class TaskStatusUpdate(BaseModel):
@@ -77,6 +78,7 @@ class TaskResponse(TaskBase):
     created_by: str
     created_at: datetime
     updated_at: datetime
+    version: int = 1  # Optimistic locking version
 
     class Config:
         from_attributes = True
@@ -100,3 +102,32 @@ class TaskWithDetails(TaskResponse):
 class TaskListResponse(BaseModel):
     tasks: List[TaskWithDetails]
     total: int
+
+
+class TaskRestoreRequest(BaseModel):
+    """Request body for restoring a soft-deleted task."""
+    cascade: bool = Field(
+        default=True,
+        description="If True, also restore child tasks deleted at the same time. If False, restore only the parent task."
+    )
+
+
+class TaskRestoreResponse(BaseModel):
+    """Response for task restore operation."""
+    restored_task: TaskResponse
+    restored_children_count: int = 0
+    restored_children_ids: List[str] = []
+
+
+class TaskDeleteWarningResponse(BaseModel):
+    """Response when task has unresolved blockers and force_delete is False."""
+    warning: str
+    blocker_count: int
+    message: str = "Task has unresolved blockers. Use force_delete=true to delete anyway."
+
+
+class TaskDeleteResponse(BaseModel):
+    """Response for task delete operation."""
+    task: TaskResponse
+    blockers_resolved: int = 0
+    force_deleted: bool = False