Remove deprecated WebSocket API, switch to SQLite for reply store

sql/queries.sql

@@ -0,0 +1,5 @@
+/* name: ItemExists :one */
+SELECT COUNT(1) FROM replied_items WHERE item_type = ? AND id = ? AND updated_time = ?;
+
+/* name: AddItem :exec */
+INSERT OR REPLACE INTO replied_items (id, item_type, updated_time) VALUES (?, ?, ?);

sql/schema.sql

@@ -0,0 +1,6 @@
+CREATE TABLE IF NOT EXISTS replied_items (
+  id           INT  NOT NULL PRIMARY KEY,
+  item_type    TEXT NOT NULL CHECK( item_type IN ('p', 'c') ),
+  updated_time INT  NOT NULL,
+  UNIQUE(id, item_type)
+);