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/custom_fields/router.py

@@ -91,13 +91,17 @@ async def create_custom_field(
                 detail="Formula is required for formula fields",
             )
 
-        is_valid, error_msg = FormulaService.validate_formula(
+        is_valid, error_msg, cycle_path = FormulaService.validate_formula_with_details(
             field_data.formula, project_id, db
         )
         if not is_valid:
+            detail = {"message": error_msg}
+            if cycle_path:
+                detail["cycle_path"] = cycle_path
+                detail["cycle_description"] = " -> ".join(cycle_path)
             raise HTTPException(
                 status_code=status.HTTP_400_BAD_REQUEST,
-                detail=error_msg,
+                detail=detail,
             )
 
     # Get next position
@@ -229,13 +233,17 @@ async def update_custom_field(
 
     # Validate formula if updating formula field
     if field.field_type == "formula" and field_data.formula is not None:
-        is_valid, error_msg = FormulaService.validate_formula(
+        is_valid, error_msg, cycle_path = FormulaService.validate_formula_with_details(
             field_data.formula, field.project_id, db, field_id
         )
         if not is_valid:
+            detail = {"message": error_msg}
+            if cycle_path:
+                detail["cycle_path"] = cycle_path
+                detail["cycle_description"] = " -> ".join(cycle_path)
             raise HTTPException(
                 status_code=status.HTTP_400_BAD_REQUEST,
-                detail=error_msg,
+                detail=detail,
             )
 
     # Validate options if updating dropdown field