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/tree.py

@@ -1,11 +1,19 @@
 import uuid
 from datetime import datetime, timezone
-from typing import Optional, Any
+from typing import Optional, Any, TYPE_CHECKING
 from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, Index
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.core.database import Base
 
+if TYPE_CHECKING:
+    from app.models.user import User
+    from app.models.team import Team
+    from app.models.session import Session
+    from app.models.category import TreeCategory
+    from app.models.tag import TreeTag
+    from app.models.folder import UserFolder
+
 
 class Tree(Base):
     __tablename__ = "trees"
@@ -17,7 +25,16 @@ class Tree(Base):
     )
     name: Mapped[str] = mapped_column(String(255), nullable=False)
     description: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    # Legacy category field - kept for backward compatibility
+    # New code should use category_id instead
     category: Mapped[Optional[str]] = mapped_column(String(100), nullable=True, index=True)
+    # New category relationship
+    category_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("tree_categories.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True
+    )
     tree_structure: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False)
     author_id: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
@@ -50,4 +67,22 @@ class Tree(Base):
     team: Mapped[Optional["Team"]] = relationship("Team", back_populates="trees")
     sessions: Mapped[list["Session"]] = relationship("Session", back_populates="tree")
 
+    # New organization relationships
+    category_rel: Mapped[Optional["TreeCategory"]] = relationship("TreeCategory", back_populates="trees")
+    tags: Mapped[list["TreeTag"]] = relationship(
+        "TreeTag",
+        secondary="tree_tag_assignments",
+        back_populates="trees"
+    )
+    folders: Mapped[list["UserFolder"]] = relationship(
+        "UserFolder",
+        secondary="user_folder_trees",
+        back_populates="trees"
+    )
+
     # Full-text search index will be created in migration
+
+    @property
+    def tag_names(self) -> list[str]:
+        """Returns list of tag names for this tree."""
+        return [tag.name for tag in self.tags] if self.tags else []