feat: trigger step library sync on tree publish and deactivate on delete

- Call sync_steps_from_tree in update_tree whenever the tree is published
  (status transitions to 'published' or is already published and structure changes)
- Call deactivate_synced_steps_for_tree in delete_tree before db.commit()
  so the FK SET NULL does not nullify source_tree_id before the WHERE clause runs
- Fix ::jsonb cast syntax in step_sync.py (asyncpg rejects :: operator in text()
  queries; replaced with CAST(:content AS jsonb))
- Add UniqueConstraint('source_tree_id','source_node_id') to StepLibrary model
  so Base.metadata.create_all (used by tests) creates the constraint that the
  ON CONFLICT clause in sync_steps_from_tree depends on

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

backend/app/core/step_sync.py

@@ -136,7 +136,7 @@ async def sync_steps_from_tree(
                     helpful_yes, helpful_no, is_featured, is_verified,
                     created_at, updated_at
                 ) VALUES (
-                    gen_random_uuid(), :title, :step_type, :content::jsonb,
+                    gen_random_uuid(), :title, :step_type, CAST(:content AS jsonb),
                     :created_by, :account_id, :visibility, true,
                     :source_tree_id, :source_node_id, :last_synced_at,
                     '{}', true,