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/trees.ts

@@ -1,23 +1,8 @@
 import apiClient from './client'
-import type { Tree, TreeListItem, TreeCreate, TreeUpdate } from '@/types'
-
-export interface TreeListParams {
-  page?: number
-  size?: number
-  category?: string
-  include_inactive?: boolean
-}
-
-export interface TreeListResponse {
-  items: TreeListItem[]
-  total: number
-  page: number
-  size: number
-  pages: number
-}
+import type { Tree, TreeListItem, TreeCreate, TreeUpdate, TreeFilters } from '@/types'
 
 export const treesApi = {
-  async list(params?: TreeListParams): Promise<TreeListItem[]> {
+  async list(params?: TreeFilters): Promise<TreeListItem[]> {
     const response = await apiClient.get<TreeListItem[]>('/trees', { params })
     return response.data
   },
@@ -41,14 +26,15 @@ export const treesApi = {
     await apiClient.delete(`/trees/${id}`)
   },
 
-  async categories(): Promise<string[]> {
+  // Legacy categories endpoint (returns string categories)
+  async legacyCategories(): Promise<string[]> {
     const response = await apiClient.get<string[]>('/trees/categories')
     return response.data
   },
 
-  async search(query: string, category?: string): Promise<TreeListItem[]> {
+  async search(query: string, limit?: number): Promise<TreeListItem[]> {
     const response = await apiClient.get<TreeListItem[]>('/trees/search', {
-      params: { q: query, category },
+      params: { q: query, limit },
     })
     return response.data
   },