Add tree organization system with categories, tags, and folders

Features:
- Categories: Global and team-specific tree categorization (admin-managed)
- Tags: Flexible tree tagging with autocomplete (author + admin)
- User folders: Personal tree collections with subfolder support
  - Hierarchical structure (max 3 levels deep)
  - Right-click context menu for folder management
  - Cascade delete for subfolders
- Filter trees by category, tags, and folder in library view

Backend:
- New models: Category, Tag, UserFolder with relationships
- New API endpoints for categories, tags, and folders
- Tree organization migrations (005, 006)

Frontend:
- FolderSidebar with hierarchical folder tree
- FolderEditModal for create/edit with color picker
- AddToFolderMenu for quick tree organization
- TagInput with autocomplete and TagBadges display
- Updated TreeMetadataForm and TreeLibraryPage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/folders.ts

@@ -0,0 +1,50 @@
+import apiClient from './client'
+import type { Folder, FolderListItem, FolderCreate, FolderUpdate, FolderReorderRequest } from '@/types'
+
+export const foldersApi = {
+  async list(): Promise<FolderListItem[]> {
+    const response = await apiClient.get<FolderListItem[]>('/folders')
+    return response.data
+  },
+
+  async get(id: string): Promise<Folder> {
+    const response = await apiClient.get<Folder>(`/folders/${id}`)
+    return response.data
+  },
+
+  async create(data: FolderCreate): Promise<Folder> {
+    const response = await apiClient.post<Folder>('/folders', data)
+    return response.data
+  },
+
+  async update(id: string, data: FolderUpdate): Promise<Folder> {
+    const response = await apiClient.put<Folder>(`/folders/${id}`, data)
+    return response.data
+  },
+
+  async delete(id: string): Promise<void> {
+    await apiClient.delete(`/folders/${id}`)
+  },
+
+  async reorder(folderIds: string[]): Promise<void> {
+    await apiClient.post('/folders/reorder', {
+      folder_ids: folderIds,
+    } as FolderReorderRequest)
+  },
+
+  // Folder tree management
+  async getTreeIds(folderId: string): Promise<string[]> {
+    const response = await apiClient.get<string[]>(`/folders/${folderId}/trees`)
+    return response.data
+  },
+
+  async addTree(folderId: string, treeId: string): Promise<void> {
+    await apiClient.post(`/folders/${folderId}/trees`, { tree_id: treeId })
+  },
+
+  async removeTree(folderId: string, treeId: string): Promise<void> {
+    await apiClient.delete(`/folders/${folderId}/trees/${treeId}`)
+  },
+}
+
+export default foldersApi