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

docs/api/openapi.yaml

@@ -4323,6 +4323,19 @@ paths:
             maxLength: 2048
           description: Opaque multi-domain keyset cursor from the previous page. Treat as fully opaque — echo, never parse. Omit
             for the initial sync.
+        - in: query
+          name: start
+          schema:
+            description: "`now` answers an empty page (every domain empty, `hasMore: false`) whose `cursor` sits at `serverNow` in
+              every domain, in the same opaque format as any other page. Paging forward from it yields only what changes
+              from then on, including `measurementMirror`, without walking the history first. Mutually exclusive with
+              `cursor` (422). A server without it ignores the parameter and starts from the oldest row."
+            type: string
+            const: now
+          description: "`now` answers an empty page (every domain empty, `hasMore: false`) whose `cursor` sits at `serverNow` in
+            every domain, in the same opaque format as any other page. Paging forward from it yields only what changes
+            from then on, including `measurementMirror`, without walking the history first. Mutually exclusive with
+            `cursor` (422). A server without it ignores the parameter and starts from the oldest row."
         - in: query
           name: limit
           schema: