feat(knowledge): add KnowledgeItem model

lib/data/models/knowledge_item.dart

@@ -0,0 +1,51 @@
+class KnowledgeItem {
+  final int id;
+  final String noteType; // 'note' | 'person' | 'place' | 'list' | 'task'
+  final String title;
+  final String body;
+  final List<String> tags;
+  final int? projectId;
+  final int? milestoneId;
+  final int? parentId;
+  // Task-only fields (null for non-tasks)
+  final String? status;   // 'todo' | 'in_progress' | 'done' | 'cancelled'
+  final String? priority; // 'low' | 'normal' | 'high'
+  final String? dueDate;
+  final DateTime createdAt;
+  final DateTime updatedAt;
+
+  const KnowledgeItem({
+    required this.id,
+    required this.noteType,
+    required this.title,
+    required this.body,
+    required this.tags,
+    this.projectId,
+    this.milestoneId,
+    this.parentId,
+    this.status,
+    this.priority,
+    this.dueDate,
+    required this.createdAt,
+    required this.updatedAt,
+  });
+
+  factory KnowledgeItem.fromJson(Map<String, dynamic> json) => KnowledgeItem(
+        id: json['id'] as int,
+        noteType: json['note_type'] as String? ?? 'note',
+        title: json['title'] as String? ?? '',
+        body: json['body'] as String? ?? '',
+        tags: (json['tags'] as List<dynamic>?)
+                ?.map((e) => e as String)
+                .toList() ??
+            [],
+        projectId: json['project_id'] as int?,
+        milestoneId: json['milestone_id'] as int?,
+        parentId: json['parent_id'] as int?,
+        status: json['status'] as String?,
+        priority: json['priority'] as String?,
+        dueDate: json['due_date'] as String?,
+        createdAt: DateTime.parse(json['created_at'] as String),
+        updatedAt: DateTime.parse(json['updated_at'] as String),
+      );
+}