feat: auto-assign strategy (round_robin/least_loaded/first_free); urgent tasks always use first_free

backend/migrations/031_auto_assign_strategy.sql

@@ -0,0 +1,3 @@
+ALTER TABLE housekeeping_settings
+  ADD COLUMN IF NOT EXISTS auto_assign          BOOLEAN     NOT NULL DEFAULT false,
+  ADD COLUMN IF NOT EXISTS auto_assign_strategy VARCHAR(20) NOT NULL DEFAULT 'least_loaded';