feat: add durable Kuknos settlement coordination

infrastructure/postgres/migrations/000003_kuknos_settlements.up.sql

@@ -0,0 +1,22 @@
+CREATE TABLE kuknos_settlements (
+    id uuid PRIMARY KEY,
+    source_service text NOT NULL CHECK (source_service <> ''),
+    source_transaction_id text NOT NULL CHECK (source_transaction_id <> ''),
+    idempotency_key text NOT NULL UNIQUE CHECK (idempotency_key <> ''),
+    status text NOT NULL CHECK (status IN ('PENDING','SUBMITTED','CONFIRMED','RETRYABLE','MANUAL_REVIEW')),
+    network text NOT NULL DEFAULT 'kuknos',
+    signed_transaction_xdr text NOT NULL DEFAULT '',
+    transaction_hash text NOT NULL DEFAULT '',
+    attempts integer NOT NULL DEFAULT 0 CHECK (attempts >= 0),
+    available_at timestamptz NOT NULL DEFAULT clock_timestamp(),
+    locked_at timestamptz,
+    locked_by text NOT NULL DEFAULT '',
+    last_error text NOT NULL DEFAULT '',
+    submitted_at timestamptz,
+    confirmed_at timestamptz,
+    manual_review_at timestamptz,
+    created_at timestamptz NOT NULL DEFAULT clock_timestamp(),
+    updated_at timestamptz NOT NULL DEFAULT clock_timestamp()
+);
+
+CREATE INDEX kuknos_settlements_due_idx ON kuknos_settlements (status, available_at);