feat: enterprise notification & task management system

Phase N.1 — Data Model:
- Extend Notification model with category, priority, task fields (status, action,
  assignee, dueDate, completedAt/By), reminder fields (remindAt, recurrence,
  nextRemindAt), and targeting metadata (sourceId, senderId, channel)
- Add NotificationCategory, NotificationPriority, TaskStatus enums
- Add NotificationBroadcast model for group notifications
- Shared types with parseTaskAction()/buildTaskAction() helpers

Phase N.2 — API:
- Extend notification router: listTasks, taskCounts, updateTaskStatus,
  createReminder/update/delete/list, createBroadcast/listBroadcasts,
  createTask, assignTask, delete
- Broadcast targeting: resolve recipients by user/role/project/orgUnit/all
- Task-action registry: approve_vacation, reject_vacation, confirm_assignment
- Reminder scheduler: 60s poll interval, recurring support, catch-up on start
- SSE events: TASK_ASSIGNED, TASK_COMPLETED, TASK_STATUS_CHANGED,
  REMINDER_DUE, BROADCAST_SENT

Phase N.3 — AI Assistant:
- 7 new tools: list_tasks, get_task_detail, update_task_status,
  execute_task_action, create_reminder, create_task_for_user, send_broadcast
- execute_task_action dispatches to task-action registry with per-action
  permission checks, marks tasks as completed by AI

Phase N.4 — Frontend:
- Enhanced NotificationBell with task badge, tabs (All/Tasks/Reminders)
- TaskCard component with priority badges, due dates, action buttons
- ReminderModal for creating/editing personal reminders
- BroadcastModal for targeted group notifications (manager+)
- NotificationCenter full-page with 5 tabs and bulk actions
- TaskWidget dashboard widget showing open tasks
- Admin broadcast management page
- AppShell nav links for Notifications and Broadcasts
- SSE hook handlers for task/reminder events

Phase N.5 — Auto-Tasks:
- Vacation create → APPROVAL tasks for all managers
- Vacation approve/reject → mark approval tasks as DONE
- Demand create → TASK for managers to fill staffing needs

Co-Authored-By: claude-flow <ruv@ruv.net>

packages/db/prisma/schema.prisma

@@ -185,6 +185,9 @@ model User {
   vacationsApproved  Vacation[] @relation("vacation_approved")
   resource           Resource?
   notifications      Notification[]
+  tasksAssigned      Notification[]          @relation("taskAssignee")
+  notificationsSent  Notification[]          @relation("notificationSender")
+  broadcasts         NotificationBroadcast[] @relation("broadcastSender")
 
   createdAt         DateTime  @default(now())
   updatedAt         DateTime  @updatedAt
@@ -1286,6 +1289,29 @@ model VacationEntitlement {
   @@map("vacation_entitlements")
 }
 
+// ─── Notification Enums ──────────────────────────────────────────────────────
+
+enum NotificationCategory {
+  NOTIFICATION
+  REMINDER
+  TASK
+  APPROVAL
+}
+
+enum NotificationPriority {
+  LOW
+  NORMAL
+  HIGH
+  URGENT
+}
+
+enum TaskStatus {
+  OPEN
+  IN_PROGRESS
+  DONE
+  DISMISSED
+}
+
 // ─── Notification ─────────────────────────────────────────────────────────────
 
 model Notification {
@@ -1299,12 +1325,71 @@ model Notification {
   readAt     DateTime?
   createdAt  DateTime  @default(now())
 
-  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+  // Extended fields
+  category   NotificationCategory @default(NOTIFICATION)
+  priority   NotificationPriority @default(NORMAL)
+  link       String?
+
+  // Task fields
+  taskStatus   TaskStatus?
+  taskAction   String?
+  assigneeId   String?
+  dueDate      DateTime?
+  completedAt  DateTime?
+  completedBy  String?
+
+  // Reminder fields
+  remindAt     DateTime?
+  recurrence   String?      // "daily" | "weekly" | "monthly" | null
+  nextRemindAt DateTime?
+
+  // Targeting
+  sourceId     String?
+  senderId     String?
+  channel      String  @default("in_app")
+
+  updatedAt    DateTime @updatedAt
+
+  // Relations
+  user     User  @relation(fields: [userId], references: [id], onDelete: Cascade)
+  assignee User? @relation("taskAssignee", fields: [assigneeId], references: [id])
+  sender   User? @relation("notificationSender", fields: [senderId], references: [id])
 
   @@index([userId, readAt])
+  @@index([userId, category, taskStatus])
+  @@index([nextRemindAt])
+  @@index([assigneeId, taskStatus])
   @@map("notifications")
 }
 
+// ─── Notification Broadcast ─────────────────────────────────────────────────
+
+model NotificationBroadcast {
+  id             String               @id @default(cuid())
+  senderId       String
+  title          String
+  body           String?
+  link           String?
+  category       NotificationCategory @default(NOTIFICATION)
+  priority       NotificationPriority @default(NORMAL)
+  channel        String               @default("in_app")
+
+  targetType     String   // "user" | "role" | "project" | "orgUnit" | "all"
+  targetValue    String?
+
+  scheduledAt    DateTime?
+  sentAt         DateTime?
+  recipientCount Int @default(0)
+
+  createdAt      DateTime @default(now())
+
+  sender User @relation("broadcastSender", fields: [senderId], references: [id])
+
+  @@index([senderId])
+  @@index([scheduledAt, sentAt])
+  @@map("notification_broadcasts")
+}
+
 // ─── System Settings ──────────────────────────────────────────────────────────
 
 model SystemSettings {