fix(sync): place a start=now cursor a minute behind the present

updatedAt is stamped when a write is built, not when it commits, so a
transaction in flight when the cursor was handed out could commit a row
stamped before it, and a watermark at serverNow skipped that row for
good. The watermarks now sit one minute back; client upserts are
idempotent, so the overlap is harmless. The token's issue time stays
serverNow.

docs/api/openapi.yaml

@@ -4515,16 +4515,20 @@ paths:
         - 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."
+            description: "`now` answers an empty page (every domain empty, `hasMore: false`) whose `cursor` sits one minute before
+              `serverNow` in every domain, in the same opaque format as any other page. Paging forward from it yields
+              what changes from then on, plus anything written in that last minute (a write stamped before `serverNow`
+              can commit after it, and is not skipped; upserts are idempotent, so a row seen twice is harmless),
+              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."
+          description: "`now` answers an empty page (every domain empty, `hasMore: false`) whose `cursor` sits one minute before
+            `serverNow` in every domain, in the same opaque format as any other page. Paging forward from it yields what
+            changes from then on, plus anything written in that last minute (a write stamped before `serverNow` can
+            commit after it, and is not skipped; upserts are idempotent, so a row seen twice is harmless), 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: