feat(sync): let a client start the delta feed at the present

GET /api/sync/changes?start=now answers an empty page whose cursor sits at
serverNow in every domain, in the same opaque format the feed hands out.
A client that only needs what changes from now on, such as the Apple
Health write-back, pages forward from it instead of draining the whole
history first. start and cursor together are refused with 422.

Refs #1339

src/lib/sync/changes/pull.ts

@@ -13,6 +13,7 @@ import {
   decodeCursor,
   decodeCursorIssuedAt,
   encodeCursor,
+  encodeCursorAtNow,
   SYNC_DOMAINS,
   type SyncCursor,
   type SyncDomain,
@@ -108,6 +109,26 @@ function emptyChanges(): SyncChangesBody["changes"] {
   };
 }
 
+/**
+ * `start=now` (#1339): an empty page with a cursor at `serverNow` in every
+ * domain, for a client that wants changes from now on (the Apple Health
+ * write-back) without paging through the history first. `hasMore` is false:
+ * the client is caught up as of `serverNow` by definition.
+ */
+export function syncChangesFromNow(serverNow: Date): SyncChangesBody {
+  annotate({
+    action: { name: "sync.changes.start_now" },
+    meta: { returned: 0 },
+  });
+  return {
+    serverNow: serverNow.toISOString(),
+    cursor: encodeCursorAtNow(serverNow.getTime()),
+    hasMore: false,
+    cursorExpired: false,
+    changes: emptyChanges(),
+  };
+}
+
 export async function pullSyncChanges(input: {
   userId: string;
   /** The opaque cursor the client echoed, if any. */