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/team.py

@@ -1,10 +1,17 @@
 import uuid
 from datetime import datetime, timezone
+from typing import TYPE_CHECKING
 from sqlalchemy import String, DateTime
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID
 from app.core.database import Base
 
+if TYPE_CHECKING:
+    from app.models.user import User
+    from app.models.tree import Tree
+    from app.models.category import TreeCategory
+    from app.models.tag import TreeTag
+
 
 class Team(Base):
     __tablename__ = "teams"
@@ -23,3 +30,5 @@ class Team(Base):
     # Relationships
     users: Mapped[list["User"]] = relationship("User", back_populates="team")
     trees: Mapped[list["Tree"]] = relationship("Tree", back_populates="team")
+    categories: Mapped[list["TreeCategory"]] = relationship("TreeCategory", back_populates="team")
+    tags: Mapped[list["TreeTag"]] = relationship("TreeTag", back_populates="team")