feat: add durable user message queue storage

codex-rs/state/queue_migrations/0001_queued_items.sql

@@ -0,0 +1,18 @@
+CREATE TABLE queued_items (
+    queued_item_id TEXT PRIMARY KEY NOT NULL,
+    thread_id TEXT NOT NULL,
+    payload_jsonb BLOB NOT NULL,
+    queue_order INTEGER NOT NULL,
+    state TEXT NOT NULL CHECK (state IN ('pending', 'claimed', 'failed')),
+    claim_token TEXT,
+    failure_jsonb BLOB,
+    created_at_ms INTEGER NOT NULL,
+    updated_at_ms INTEGER NOT NULL,
+    CHECK ((state = 'claimed') = (claim_token IS NOT NULL))
+);
+
+CREATE INDEX queued_items_thread_state_order_idx
+    ON queued_items(thread_id, state, queue_order);
+
+CREATE INDEX queued_items_thread_order_idx
+    ON queued_items(thread_id, queue_order);