feat(postgres): failed_events table and PgFailedEventStore for dead-letter queue

crates/adapters/postgres/migrations/009_failed_events.sql

@@ -0,0 +1,15 @@
+CREATE TABLE failed_events (
+    id           UUID        NOT NULL DEFAULT gen_random_uuid(),
+    event_type   TEXT        NOT NULL,
+    payload      JSONB       NOT NULL,
+    failed_at    TIMESTAMPTZ NOT NULL DEFAULT now(),
+    retry_at     TIMESTAMPTZ NOT NULL,
+    retry_count  INT         NOT NULL DEFAULT 0,
+    last_error   TEXT        NOT NULL,
+
+    CONSTRAINT failed_events_pkey PRIMARY KEY (id)
+);
+
+CREATE INDEX failed_events_due_idx
+    ON failed_events (retry_at)
+    WHERE retry_count < 3;