TICKET-006 (backend): OFF normalization, scan/search/refresh, restore-on-rescan

- services/off.py: single normalizer module (kcal/kJ fallback, not-found
  rule); fetch_product/search_off degrade gracefully on upstream errors
  (503/timeout → None / []), never 500; User-Agent + timeouts on all calls
- routers/off.py: thin routes for GET /api/off/product/{barcode},
  GET /api/off/search, POST /api/off/refresh/{food_id} (404 unknown id,
  400 no-barcode)
- services/foods.py: restore-on-rescan (§3.1) — barcode collision on a
  soft-deleted food clears deleted_at + updates row instead of 409;
  GET /api/foods/recent ordered by most-recent daily_log appearance,
  deduped, soft-deleted excluded
- tests: httpx mocked at the boundary (MockTransport, no real OFF);
  48 new tests (normalizer unit + router + restore + recent + error paths)
- 152 passing (was 104)

backend/tests/conftest.py

@@ -3,6 +3,7 @@
 import os
 import tempfile
 
+import httpx
 import pytest
 
 # Must be set before any app module is imported (engine binds at import time)
@@ -20,3 +21,34 @@ def client():
     run_migrations()
     with TestClient(app) as c:
         yield c
+
+
+@pytest.fixture
+def off_mock(monkeypatch):
+    """Replace _default_client() in services.off with a mock client backed
+    by httpx.MockTransport. Tests assign off_mock.handler to control
+    responses.
+
+    Usage:
+        def test_foo(client, off_mock):
+            def handler(request):
+                return httpx.Response(200, json={...})
+            off_mock.handler = handler
+            resp = client.get("/api/off/product/123")
+    """
+    state = _MockState()
+
+    def _make_mock_client():
+        return httpx.Client(
+            transport=httpx.MockTransport(lambda req: state.handler(req))
+        )
+
+    import services.off as off_mod
+    monkeypatch.setattr(off_mod, "_default_client", _make_mock_client)
+    return state
+
+
+class _MockState:
+    """Mutable state so tests can set .handler after fixture injection."""
+    def __init__(self):
+        self.handler = None