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>

backend/app/models/__init__.py

@@ -4,5 +4,20 @@ from .tree import Tree
 from .session import Session
 from .attachment import Attachment
 from .invite_code import InviteCode
+from .category import TreeCategory
+from .tag import TreeTag, tree_tag_assignments
+from .folder import UserFolder, user_folder_trees
 
-__all__ = ["User", "Team", "Tree", "Session", "Attachment", "InviteCode"]
+__all__ = [
+    "User",
+    "Team",
+    "Tree",
+    "Session",
+    "Attachment",
+    "InviteCode",
+    "TreeCategory",
+    "TreeTag",
+    "tree_tag_assignments",
+    "UserFolder",
+    "user_folder_trees",
+]