Add is_default flag for system trees

- Add is_default column to trees table
- Default trees have no author and are visible to all users
- Only admins can create default trees
- Update seed script to mark seeded trees as default
- Update seed script to use CLI auth instead of creating seed user

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/alembic/versions/003_add_tree_is_default.py

@@ -0,0 +1,28 @@
+"""Add is_default field to trees
+
+Revision ID: 003
+Revises: 002
+Create Date: 2026-02-01
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '003'
+down_revision: Union[str, None] = '002'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('trees', sa.Column('is_default', sa.Boolean(), nullable=False, server_default='false'))
+    op.create_index('ix_trees_is_default', 'trees', ['is_default'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_trees_is_default', table_name='trees')
+    op.drop_column('trees', 'is_default')