refactor(search): address PR #750 round 5 review feedback

Tightens verifier consistency, closes test gaps, hardens the fire-and-forget
eviction snapshot, and routes the new concurrency knob through Settings.

- Pre-flight ``int()`` guard in ``_verify_notes`` mirrors ``_verify_deck_cards``,
  so a non-numeric note id produces a type-specific log line instead of
  falling through to the generic "unexpected error" branch.
- Adds explicit 403 tests for the file and news verifiers (symmetry with the
  existing notes/deck 403 tests) plus a ``non_numeric_id_keeps`` test.
- ``AppContext`` and ``OAuthAppContext`` no longer snapshot
  ``_vector_sync_state.eviction_task_group`` at lifespan-yield time. Both
  expose it as a ``@property`` that reads the singleton dynamically, removing
  the order-sensitive race where a future startup-ordering change could
  silently degrade fire-and-forget eviction to inline forever.
- Adds ``verification_concurrency`` (env var ``VERIFICATION_CONCURRENCY``,
  default 20) to ``Settings`` with a dynaconf validator; ``verify_search_results``
  resolves the cap lazily from settings when the caller doesn't override it.
- Enriches the news verifier TODO to call out that ``batch_size=-1`` is
  intentional — a numeric ceiling would silently break correctness because
  any item beyond the cap would be missing from ``present_ids`` and dropped.
- Updates ``Optional[TaskGroup]`` to ``TaskGroup | None`` per project style.

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

docs/configuration.md

@@ -489,10 +489,11 @@ aware of:
   unique documents, so verification adds 3-5 round-trips. With the default
   20-way concurrency this is one parallel batch — usually under 100 ms on a
   healthy connection.
-- **Concurrency**: all verifications fan out under a shared semaphore
-  (`DEFAULT_VERIFICATION_CONCURRENCY = 20` in `search/verification.py`). The
-  limit is not currently exposed as an env var; if production workloads
-  saturate Nextcloud, consider opening an issue to make it tunable.
+- **Concurrency**: all verifications fan out under a shared semaphore.
+  Tunable via the `VERIFICATION_CONCURRENCY` env var (settings field
+  `verification_concurrency`, default 20) — lower it if your Nextcloud
+  backend struggles with the parallel fan-out, or raise it on a healthy
+  connection to speed up large result pages.
 - **News API caveat**: the News app has no per-item endpoint, so the news
   verifier issues a single `news.get_items(batch_size=-1, get_read=True)` call
   per search that contains any news result, then intersects locally. The