fix(vector-sync): address PR review — dict guard, symmetric backstop, metrics

- purge route: 400 (not 500) on a valid-JSON non-object body
- scanner: backstop-purge admin-disabled note/news_item/deck_card points
  (their deletion-tracking lives inside the skipped scan_* fns), mirroring the
  files path; gated on a concrete allow-set so fail-open never deletes
- processor: record_ingest_dropped("admin_disabled") so consent-skipped index
  tasks are observable/alertable
- app.py: list /api/v1/vector-sync/purge in the endpoints log line
- capabilities: drop empty-string doc types; return frozenset throughout
- purge: document the count-before-delete approximation
- tests: non-object body -> 400, ProvisioningRequiredError -> 428, cache TTL
  expiry refetch, and the scanner consent backstop

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/vector/test_purge.py

@@ -20,10 +20,12 @@ def _patch_qdrant(monkeypatch, *, counts: dict[str, int], delete_raises=None):
     def _doc_type_of(flt):
         return flt.must[0].match.value
 
-    async def fake_count(*, collection_name, count_filter, exact):
+    # Sync side_effects: AsyncMock awaits the call and returns the value, so the
+    # helpers don't need to be coroutines themselves.
+    def fake_count(*, collection_name, count_filter, exact):
         return SimpleNamespace(count=counts.get(_doc_type_of(count_filter), 0))
 
-    async def fake_delete(*, collection_name, points_selector):
+    def fake_delete(*, collection_name, points_selector):
         dt = _doc_type_of(points_selector)
         if delete_raises and dt in delete_raises:
             raise RuntimeError(f"delete failed for {dt}")
@@ -31,10 +33,9 @@ def _patch_qdrant(monkeypatch, *, counts: dict[str, int], delete_raises=None):
     client.count.side_effect = fake_count
     client.delete.side_effect = fake_delete
 
-    async def fake_get_qdrant_client():
-        return client
-
-    monkeypatch.setattr(purge_module, "get_qdrant_client", fake_get_qdrant_client)
+    monkeypatch.setattr(
+        purge_module, "get_qdrant_client", AsyncMock(return_value=client)
+    )
     monkeypatch.setattr(
         purge_module,
         "get_settings",