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/src/components/TaskDetailModal.tsx

@@ -26,6 +26,7 @@ interface Task {
   subtask_count: number
   parent_task_id: string | null
   custom_values?: CustomValueResponse[]
+  version?: number
 }
 
 interface TaskStatus {
@@ -52,9 +53,14 @@ export function TaskDetailModal({
   onUpdate,
   onSubtaskClick,
 }: TaskDetailModalProps) {
-  const { t } = useTranslation('tasks')
+  const { t, i18n } = useTranslation('tasks')
   const [isEditing, setIsEditing] = useState(false)
   const [saving, setSaving] = useState(false)
+  const [conflictError, setConflictError] = useState<{
+    message: string
+    currentVersion: number
+    yourVersion: number
+  } | null>(null)
   const [editForm, setEditForm] = useState({
     title: task.title,
     description: task.description || '',
@@ -153,6 +159,7 @@ export function TaskDetailModal({
 
   const handleSave = async () => {
     setSaving(true)
+    setConflictError(null)
     try {
       const payload: Record<string, unknown> = {
         title: editForm.title,
@@ -160,6 +167,11 @@ export function TaskDetailModal({
         priority: editForm.priority,
       }
 
+      // Include version for optimistic locking
+      if (task.version) {
+        payload.version = task.version
+      }
+
       // Always send status_id (null to clear, or the value)
       payload.status_id = editForm.status_id || null
       // Always send assignee_id (null to clear, or the value)
@@ -194,13 +206,34 @@ export function TaskDetailModal({
       await api.patch(`/tasks/${task.id}`, payload)
       setIsEditing(false)
       onUpdate()
-    } catch (err) {
+    } catch (err: unknown) {
+      // Handle 409 Conflict - version mismatch
+      if (err && typeof err === 'object' && 'response' in err) {
+        const axiosError = err as { response?: { status?: number; data?: { detail?: { error?: string; message?: string; current_version?: number; your_version?: number } } } }
+        if (axiosError.response?.status === 409) {
+          const detail = axiosError.response?.data?.detail
+          if (detail?.error === 'conflict') {
+            setConflictError({
+              message: detail.message || t('conflict.message'),
+              currentVersion: detail.current_version || 0,
+              yourVersion: detail.your_version || 0,
+            })
+            return
+          }
+        }
+      }
       console.error('Failed to update task:', err)
     } finally {
       setSaving(false)
     }
   }
 
+  const handleRefreshTask = () => {
+    setConflictError(null)
+    setIsEditing(false)
+    onUpdate()
+  }
+
   const handleCustomFieldChange = (fieldId: string, value: string | number | null) => {
     setEditCustomValues((prev) => ({
       ...prev,
@@ -289,6 +322,22 @@ export function TaskDetailModal({
           </div>
         </div>
 
+        {/* Conflict Error Banner */}
+        {conflictError && (
+          <div style={styles.conflictBanner}>
+            <div style={styles.conflictContent}>
+              <div style={styles.conflictIcon}>!</div>
+              <div style={styles.conflictText}>
+                <div style={styles.conflictTitle}>{t('conflict.title')}</div>
+                <div style={styles.conflictMessage}>{t('conflict.message')}</div>
+              </div>
+            </div>
+            <button onClick={handleRefreshTask} style={styles.conflictRefreshButton}>
+              {t('common:buttons.refresh')}
+            </button>
+          </div>
+        )}
+
         <div style={styles.content}>
           <div style={styles.mainSection}>
             {/* Description */}
@@ -424,7 +473,11 @@ export function TaskDetailModal({
               ) : (
                 <div style={styles.dueDateDisplay}>
                   {task.due_date
-                    ? new Date(task.due_date).toLocaleDateString()
+                    ? new Date(task.due_date).toLocaleDateString(i18n.language, {
+                        year: 'numeric',
+                        month: 'short',
+                        day: 'numeric',
+                      })
                     : t('status.noDueDate')}
                 </div>
               )}
@@ -742,6 +795,54 @@ const styles: Record<string, React.CSSProperties> = {
     color: '#888',
     fontStyle: 'italic',
   },
+  conflictBanner: {
+    display: 'flex',
+    alignItems: 'center',
+    justifyContent: 'space-between',
+    padding: '12px 24px',
+    backgroundColor: '#fff3cd',
+    borderBottom: '1px solid #ffc107',
+  },
+  conflictContent: {
+    display: 'flex',
+    alignItems: 'center',
+    gap: '12px',
+  },
+  conflictIcon: {
+    width: '24px',
+    height: '24px',
+    borderRadius: '50%',
+    backgroundColor: '#ff9800',
+    color: 'white',
+    display: 'flex',
+    alignItems: 'center',
+    justifyContent: 'center',
+    fontWeight: 'bold',
+    fontSize: '14px',
+  },
+  conflictText: {
+    flex: 1,
+  },
+  conflictTitle: {
+    fontWeight: 600,
+    fontSize: '14px',
+    color: '#856404',
+  },
+  conflictMessage: {
+    fontSize: '13px',
+    color: '#856404',
+    marginTop: '2px',
+  },
+  conflictRefreshButton: {
+    padding: '8px 16px',
+    backgroundColor: '#ff9800',
+    color: 'white',
+    border: 'none',
+    borderRadius: '4px',
+    cursor: 'pointer',
+    fontSize: '14px',
+    fontWeight: 500,
+  },
 }
 
 export default TaskDetailModal