feat: report the source export timestamp on every travel export page

docs/api-consumer-guide.md

@@ -202,6 +202,7 @@ an error: it returns `200` with an empty `data` array and the usual `meta`.
     "totalPages": 1,
     "skipped": 0,
     "pickupsWithoutMasterData": 0,
+    "sourceGeneratedAt": "2026-09-29T11:11:01+02:00",
     "generatedAt": "2026-09-29T12:10:06+02:00"
   }
 }
@@ -211,8 +212,8 @@ an error: it returns `200` with an empty `data` array and the usual `meta`.
 is always an int and never null; `hotel` is always an object and never null; `pickups` is always an array, possibly
 empty, never null.
 
-**`meta` describes the whole export, not the page.** `total`, `totalPages`, `skipped`, `pickupsWithoutMasterData`
-and `generatedAt` are identical on every page, so you can read them once on page 1 and decide whether to walk the
+**`meta` describes the whole export, not the page.** `total`, `totalPages`, `skipped`, `pickupsWithoutMasterData`,
+`sourceGeneratedAt` and `generatedAt` are identical on every page, so you can read them once on page 1 and decide whether to walk the
 rest. `total` is the row count **after** skips, and `totalPages` is `ceil(total / perPage)`.
 
 - `skipped` — (travel, hotel) pairs dropped as unusable: a travel date that could not be parsed, or a hotel absent
@@ -240,6 +241,11 @@ rest. `total` is the row count **after** skips, and `totalPages` is `ceil(total
 under one cache tag, so all three move together on an upstream sync rather than drifting between your calls.
 It is cached for 3 hours; `generatedAt` is when the cached copy was assembled, not when you requested it.
 
+**Page consistency.** `sourceGeneratedAt` is the timestamp of the upstream export the rows were built from — the
+same value `GET /api/last-update` reports, as ISO-8601 — or `null` if it could not be read. An upstream sync can
+replace the cached copy while you are walking the pages; if `sourceGeneratedAt` differs from the value you saw on
+page 1, you have stitched two exports together. Discard what you have and restart from page 1.
+
 ---
 
 ### Travels