Sync — watchlists, history, and ratings

const PARAMS  = `client_id=${CLIENT_ID}&app-name=my-app-name&app-version=1.0`;
const HEADERS = {
  'User-Agent':    'my-app-name/1.0',
  'Authorization': `Bearer ${ACCESS_TOKEN}`,
};

// Phase 1 — initial sync. Sequential, no date_from.
async function initialSync(localState, types = ['shows', 'movies', 'anime']) {
  for (const type of types) {
    const items = await fetch(
      `https://api.simkl.com/sync/all-items/${type}?${PARAMS}`,
      { headers: HEADERS }
    ).then(r => r.json());
    storeFullList(localState, type, items);   // overwrites your local cache for this type
  }
  const activities = await fetch(
    `https://api.simkl.com/sync/activities?${PARAMS}`,
    { headers: HEADERS }
  ).then(r => r.json());
  localState.lastSync = activities.all;
}

// Phase 2 — continuous sync. Activity check, then date_from delta.
async function sync(localState) {
  if (!localState.lastSync) {
    return initialSync(localState);   // no saved timestamp → bootstrap
  }

  const activities = await fetch(
    `https://api.simkl.com/sync/activities?${PARAMS}`,
    { headers: HEADERS }
  ).then(r => r.json());

  if (activities.all === localState.lastSync) return;   // nothing changed

  // Multi-type app: hit /sync/all-items (no path segment) to pull all types in one call.
  // Single-type app: replace `all-items` below with `all-items/anime` (or movies/shows).
  const dateFrom = encodeURIComponent(localState.lastSync);
  const delta = await fetch(
    `https://api.simkl.com/sync/all-items?date_from=${dateFrom}&${PARAMS}`,
    { headers: HEADERS }
  ).then(r => r.json());

  mergeDelta(localState, delta);                    // merge per-item, don't replace
  localState.lastSync = activities.all;
}