feat(ingest): country-focused seeding (collect domestic CN products)

Add a market-focused seeding path so the catalogue can be built from
domestic products rather than the English-heavy global default:

- adapter.fetch_by_country(country): OFF search filtered by
  countries_tags_en, sorted by unique_scans_n (most-scanned first),
  de-duplicated across pages since OFF popularity ordering is unstable.
- is_cn_gs1(code): True for GS1-China company prefixes (690-699),
  i.e. genuinely domestic items vs. imports merely sold in China.
- seed_off --country <slug> [--domestic-only] [--page-size/--max-pages]:
  e.g. 'seed_off --country china --domestic-only' loads only 69x
  barcodes.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

ingestion/tests/test_off_country.py

@@ -0,0 +1,57 @@
+import httpx
+
+from opengoods.adapters.openfoodfacts import OpenFoodFactsAdapter, is_cn_gs1
+
+
+def _adapter(handler, **kwargs):
+    client = httpx.Client(transport=httpx.MockTransport(handler))
+    return OpenFoodFactsAdapter(client=client, min_interval=0, backoff_base=0, **kwargs)
+
+
+def test_is_cn_gs1():
+    assert is_cn_gs1("6901234567892")  # GS1 China prefix
+    assert is_cn_gs1("690")
+    assert not is_cn_gs1("3017624010701")  # France
+    assert not is_cn_gs1("5449000000996")  # Belgium
+    assert not is_cn_gs1("")
+    assert not is_cn_gs1(None)
+    assert not is_cn_gs1("69")  # too short to carry a prefix digit
+
+
+def test_fetch_by_country_passes_filter_and_paginates():
+    seen_params = []
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        seen_params.append(dict(request.url.params))
+        page = int(request.url.params.get("page", "1"))
+        if page == 1:
+            return httpx.Response(
+                200,
+                json={"products": [{"code": "6901"}, {"code": "6902"}]},
+            )
+        return httpx.Response(200, json={"products": []})
+
+    adapter = _adapter(handler)
+    out = list(adapter.fetch_by_country("china", page_size=2, max_pages=5))
+    assert [p["code"] for p in out] == ["6901", "6902"]
+    assert seen_params[0]["countries_tags_en"] == "china"
+    assert seen_params[0]["sort_by"] == "unique_scans_n"
+
+
+def test_fetch_by_country_dedupes_across_pages():
+    def handler(request: httpx.Request) -> httpx.Response:
+        page = int(request.url.params.get("page", "1"))
+        if page == 1:
+            return httpx.Response(
+                200, json={"products": [{"code": "6901"}, {"code": "6902"}]}
+            )
+        if page == 2:
+            # OFF popularity ordering is unstable; a repeat appears on page 2
+            return httpx.Response(
+                200, json={"products": [{"code": "6902"}, {"code": "6903"}]}
+            )
+        return httpx.Response(200, json={"products": []})
+
+    adapter = _adapter(handler)
+    out = [p["code"] for p in adapter.fetch_by_country("china", page_size=2, max_pages=5)]
+    assert out == ["6901", "6902", "6903"]