fix: Task 9 migration — include tags in template_trees INSERT

The tags column was accidentally omitted from the is_default tree copy.
Now uses COALESCE(tags, '[]'::jsonb) to preserve source tree tags.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/versions/3a40fe11b427_create_global_content_tables.py

@@ -54,10 +54,10 @@ def upgrade() -> None:
     op.execute("""
         INSERT INTO template_trees
             (id, name, description, category, tree_type, tree_structure,
-             is_active, created_at, updated_at, source_tree_id)
+             tags, is_active, created_at, updated_at, source_tree_id)
         SELECT
             gen_random_uuid(), name, description, category, tree_type,
-            tree_structure, is_active,
+            tree_structure, COALESCE(tags, '[]'::jsonb), is_active,
             COALESCE(created_at, NOW()), COALESCE(updated_at, NOW()), id
         FROM trees
         WHERE is_default = TRUE