New Feature: Watch Listings (#215)

* adding new feature: watch listings for changes

* adding todo for watch feature

* sort by watch

lib/services/storage/watchListStorage.js

@@ -0,0 +1,64 @@
+import SqliteConnection from './SqliteConnection.js';
+import { nanoid } from 'nanoid';
+
+/**
+ * Create a watch entry. Idempotent due to unique index (listing_id, user_id).
+ * @param {string} listingId
+ * @param {string} userId
+ * @returns {{created:boolean}}
+ */
+export const createWatch = (listingId, userId) => {
+  if (!listingId || !userId) return { created: false };
+  try {
+    SqliteConnection.execute(
+      `INSERT INTO watch_list (id, listing_id, user_id)
+       VALUES (@id, @listing_id, @user_id)
+       ON CONFLICT(listing_id, user_id) DO NOTHING`,
+      { id: nanoid(), listing_id: listingId, user_id: userId },
+    );
+    // check whether it exists now
+    const row = SqliteConnection.query(
+      `SELECT 1 AS ok FROM watch_list WHERE listing_id = @listing_id AND user_id = @user_id LIMIT 1`,
+      { listing_id: listingId, user_id: userId },
+    );
+    return { created: row.length > 0 };
+  } catch {
+    return { created: false };
+  }
+};
+
+/**
+ * Delete a watch entry.
+ * @param {string} listingId
+ * @param {string} userId
+ * @returns {{deleted:boolean}}
+ */
+export const deleteWatch = (listingId, userId) => {
+  if (!listingId || !userId) return { deleted: false };
+  const res = SqliteConnection.execute(`DELETE FROM watch_list WHERE listing_id = @listing_id AND user_id = @user_id`, {
+    listing_id: listingId,
+    user_id: userId,
+  });
+  return { deleted: Boolean(res?.changes) };
+};
+
+/**
+ * Toggle a watch entry. If exists -> delete, otherwise create.
+ * @param {string} listingId
+ * @param {string} userId
+ * @returns {{watched:boolean}}
+ */
+export const toggleWatch = (listingId, userId) => {
+  if (!listingId || !userId) return { watched: false };
+  const exists =
+    SqliteConnection.query(
+      `SELECT 1 AS ok FROM watch_list WHERE listing_id = @listing_id AND user_id = @user_id LIMIT 1`,
+      { listing_id: listingId, user_id: userId },
+    ).length > 0;
+  if (exists) {
+    deleteWatch(listingId, userId);
+    return { watched: false };
+  }
+  createWatch(listingId, userId);
+  return { watched: true };
+};