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>

frontend/public/locales/en/tasks.json

@@ -181,5 +181,33 @@
     "title": "No Tasks",
     "description": "There are no tasks yet. Create your first task to get started!",
     "filtered": "No tasks match your filters"
+  },
+  "dependencies": {
+    "title": "Task Dependencies",
+    "add": "Add Dependency",
+    "remove": "Remove Dependency",
+    "predecessor": "Predecessor",
+    "successor": "Successor",
+    "type": "Dependency Type",
+    "types": {
+      "FS": "Finish-to-Start",
+      "SS": "Start-to-Start",
+      "FF": "Finish-to-Finish",
+      "SF": "Start-to-Finish"
+    },
+    "circularError": {
+      "title": "Circular Dependency Detected",
+      "description": "Adding this dependency would create a circular reference, which is not allowed.",
+      "cyclePath": "Dependency Cycle",
+      "helpText": "To resolve this issue, choose a different task as a dependency, or remove one of the existing dependencies in the cycle."
+    },
+    "error": {
+      "addFailed": "Failed to add dependency",
+      "removeFailed": "Failed to remove dependency"
+    }
+  },
+  "conflict": {
+    "title": "Update Conflict",
+    "message": "This task has been modified by another user. Please refresh to see the latest version and try again."
   }
 }