fix(vector-sync): address round-5 review — partial-failure signal, markers

- purge route: include a "failed" key in the 200 body listing requested doc
  types that were not purged, so Astrolabe knows consent isn't yet enforced
  for them (scanner backstop still catches up)
- tests: add @pytest.mark.unit / module-level pytestmark to the new test
  modules so they run under `pytest -m unit`; add a partial-failure route test
- capabilities: comment why the cache is keyed per-user despite a global value
- semantic/scanner: doc/comment clarifications (sorted-order, eviction timing)

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

nextcloud_mcp_server/vector/scanner.py

@@ -377,7 +377,10 @@ async def _enqueue_deletes_for_disabled_types(
         if len(_consent_backstop_done) >= _CONSENT_BACKSTOP_MAX:
             # Evict oldest-first down to half capacity (insertion-ordered dict),
             # so overflow re-fires the backstop for only the oldest markers
-            # rather than the whole fleet at once.
+            # rather than the whole fleet at once. Placed inside the per-doc_type
+            # loop: markers added earlier in *this* call are the newest, so they
+            # survive eviction; only genuinely old entries are dropped (and a
+            # re-fire is idempotent regardless).
             overage = len(_consent_backstop_done) - _CONSENT_BACKSTOP_MAX // 2
             logger.info(
                 "consent backstop tracking hit %d entries; evicting %d oldest",