fix: persist uncertain cancel outcomes

trex_core/adapters/persistence/sql/020_persist_cancel_request_outcome.sql

@@ -0,0 +1,18 @@
+-- dcy-041 / GitHub #44: cancellation request acceptance is distinct from
+-- final order cancellation. Persist the immediate result to prevent blind
+-- automatic retries across execution ticks and daemon restarts.
+ALTER TABLE daily_slot
+    ADD COLUMN IF NOT EXISTS cancel_state TEXT;
+
+ALTER TABLE daily_slot
+    ADD COLUMN IF NOT EXISTS cancel_requested_at TIMESTAMPTZ;
+
+ALTER TABLE daily_slot
+    DROP CONSTRAINT IF EXISTS daily_slot_cancel_state_check;
+
+ALTER TABLE daily_slot
+    ADD CONSTRAINT daily_slot_cancel_state_check
+    CHECK (
+        cancel_state IS NULL
+        OR cancel_state IN ('requested', 'rejected', 'unknown')
+    );