feat: AM-2 wire swiftly_auth.get_token() into LuckyCaliforniaScraper

fetch_category() now calls swiftly_auth.get_token() to mint a fresh
Firebase JWT on demand when no explicit bearer_token override is
pinned by tests. The cache short-circuit means the per-call mint
overhead is ~zero in the steady state.

- Removed the empty-token short-circuit; auto-mint makes it moot
- Updated _AUTH_ERROR_MESSAGE: 401-after-mint now points at the spec
  (Lucky tightening anon-auth) rather than asking for manual capture
- Replaced test_swiftly_auth_error_when_token_missing with a positive
  test that verifies fetch_category mints when bearer_token is None
- bearer_token constructor arg preserved for the 401-path test

Full suite: 92/92 green. Live verification via
scripts/spike_swiftly_ingest.py --confirm-live deferred to next step
per HANDOFF AM-2 halt boundary.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/test_swiftly_api.py

@@ -140,22 +140,31 @@ def test_swiftly_auth_error_on_401_from_api() -> None:
         with pytest.raises(SwiftlyAuthError) as excinfo:
             scraper.fetch_category("Product/meat_seafood")
 
-    assert "SWIFTLY_BEARER_TOKEN expired" in str(excinfo.value)
+    assert "Swiftly returned 401" in str(excinfo.value)
 
 
-def test_swiftly_auth_error_when_token_missing() -> None:
-    """An empty token short-circuits to SwiftlyAuthError without any HTTP call.
-
-    Force the token empty AFTER construction so the test is independent of
-    whatever ``SWIFTLY_BEARER_TOKEN`` happens to be set in the environment
-    (it WILL be set when pytest runs inside ``docker compose``).
+def test_fetch_category_mints_token_when_none_pinned() -> None:
+    """With no explicit bearer_token, fetch_category calls swiftly_auth.get_token()
+    and threads the minted JWT into the Authorization header.
     """
+    from app.scraper import lucky_ca_scraper as scraper_mod
+
     scraper = LuckyCaliforniaScraper()
-    scraper.bearer_token = ""
-    with patch.object(scraper.api_session, "get") as mock_get:
-        with pytest.raises(SwiftlyAuthError):
-            scraper.fetch_category("Product/meat_seafood")
-        mock_get.assert_not_called()
+    assert scraper.bearer_token is None  # auto-mint path
+
+    with patch.object(scraper_mod, "_get_swiftly_token", return_value="minted-jwt") as mtoken, \
+        patch.object(
+            scraper.api_session,
+            "get",
+            return_value=_mock_response(
+                200, payload={"products": {"items": []}}
+            ),
+        ) as mget:
+        scraper.fetch_category("Product/meat_seafood")
+
+    mtoken.assert_called_once()
+    sent_headers = mget.call_args.kwargs["headers"]
+    assert sent_headers["Authorization"] == "Bearer minted-jwt"
 
 
 @pytest.mark.requires_postgres