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/alembic/versions/006_add_folder_hierarchy.py

@@ -0,0 +1,54 @@
+"""Add folder hierarchy support
+
+Revision ID: 006
+Revises: 005
+Create Date: 2026-02-02
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+
+# revision identifiers, used by Alembic.
+revision: str = '006'
+down_revision: Union[str, None] = '005'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add parent_id column for folder hierarchy
+    op.add_column(
+        'user_folders',
+        sa.Column(
+            'parent_id',
+            postgresql.UUID(as_uuid=True),
+            sa.ForeignKey('user_folders.id', ondelete='CASCADE'),
+            nullable=True
+        )
+    )
+    op.create_index('ix_user_folders_parent_id', 'user_folders', ['parent_id'])
+
+    # Update unique constraint to allow same name in different parent folders
+    # Old constraint: (user_id, name) must be unique
+    # New constraint: (user_id, name, parent_id) must be unique
+    # This allows folders with same name under different parents
+    op.drop_constraint('uq_user_folders_user_name', 'user_folders', type_='unique')
+    op.create_unique_constraint(
+        'uq_user_folders_user_name_parent',
+        'user_folders',
+        ['user_id', 'name', 'parent_id']
+    )
+
+
+def downgrade() -> None:
+    # Restore original unique constraint
+    op.drop_constraint('uq_user_folders_user_name_parent', 'user_folders', type_='unique')
+    op.create_unique_constraint('uq_user_folders_user_name', 'user_folders', ['user_id', 'name'])
+
+    # Remove parent_id column and index
+    op.drop_index('ix_user_folders_parent_id', table_name='user_folders')
+    op.drop_column('user_folders', 'parent_id')