feat: add durable Kuknos settlement coordination

infrastructure/postgres/migrations/000004_settlement_operator_audit.up.sql

@@ -0,0 +1,13 @@
+CREATE TABLE kuknos_settlement_operator_audits (
+    id bigserial PRIMARY KEY,
+    settlement_id uuid NOT NULL REFERENCES kuknos_settlements(id),
+    actor_id text NOT NULL CHECK (actor_id <> ''),
+    action text NOT NULL CHECK (action IN ('RETRY')),
+    reason text NOT NULL CHECK (reason <> ''),
+    previous_status text NOT NULL,
+    resulting_status text NOT NULL,
+    created_at timestamptz NOT NULL DEFAULT clock_timestamp()
+);
+
+CREATE INDEX kuknos_settlement_operator_audits_settlement_idx
+    ON kuknos_settlement_operator_audits (settlement_id, created_at DESC);