refactor(search): address PR #750 round 12 review feedback

Six review items raised; four required code changes (#3, #4, #5, #6) and
two were resolved without code changes (#1 audit-only, #2 informational).

* search/verification.py — clarify the granularity asymmetry between the
  whole-batch fail-open (structural API failure) and the per-item fail-open
  (single bad stored doc_id). Future readers no longer need to derive why
  the two paths have different blast radii from the code alone.

* models/semantic.py — `dropped_document_count` description now explicitly
  notes that subtracting it from `verified_chunk_count` is not a meaningful
  operation, since the two fields count different units (documents vs
  chunks). Surfaces the unit mismatch where MCP clients actually see it.

* server/semantic.py — clarify the per-doc_type over-fetch comment so the
  N×2 pre-merge Qdrant cost (vs the cross-app branch's 1×2) is explicit
  rather than implied by "same 2× over-fetch budget".

* tests/unit/search/test_verification.py — add four new 429 unit tests
  (notes/news/files/deck) mirroring the existing 5xx-keeps pattern. Locks
  in that `_is_definitive_404_or_403` returns False for 429 so a future
  refactor cannot accidentally treat rate-limit responses as permanent
  revocations.

Audit confirmation for review item #1: all four `WebDAVClient.get_file_info`
call sites already handle the new `HTTPStatusError`-on-404 contract
(verification.py:156, tests/integration/test_rag.py:139,
tests/unit/client/test_webdav.py:153/190). No silent breakage internal to
this repo.

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

tests/unit/search/test_verification.py

@@ -125,6 +125,24 @@ async def test_verify_notes_transient_5xx_keeps(mocker):
     assert result == {42}
 
 
+@pytest.mark.unit
+async def test_verify_notes_429_keeps_as_transient(mocker):
+    """HTTP 429 (rate-limit) is transient, NOT a definitive 403/404 drop.
+
+    Locks in that ``_is_definitive_404_or_403`` returns False for 429 so a
+    future refactor cannot accidentally treat rate-limit responses as
+    permanent revocations and shrink result pages on every Nextcloud hiccup.
+    """
+    notes_client = SimpleNamespace(
+        get_note=mocker.AsyncMock(side_effect=_http_error(429))
+    )
+    client = SimpleNamespace(notes=notes_client, username="alice")
+
+    result = await _verify_notes(client, [_make_result(42)], _sem())
+
+    assert result == {42}
+
+
 @pytest.mark.unit
 async def test_verify_notes_unexpected_exception_keeps(mocker):
     notes_client = SimpleNamespace(
@@ -289,6 +307,27 @@ async def test_verify_news_items_transient_keeps_all(mocker):
     assert result == {1, 2, 3}
 
 
+@pytest.mark.unit
+async def test_verify_news_items_429_keeps_as_transient(mocker):
+    """HTTP 429 from get_items must NOT collapse the batch (transient)."""
+    news_client = SimpleNamespace(
+        get_items=mocker.AsyncMock(side_effect=_http_error(429))
+    )
+    client = SimpleNamespace(news=news_client, username="alice")
+
+    result = await _verify_news_items(
+        client,
+        [
+            _make_result(1, doc_type="news_item"),
+            _make_result(2, doc_type="news_item"),
+            _make_result(3, doc_type="news_item"),
+        ],
+        _sem(),
+    )
+
+    assert result == {1, 2, 3}
+
+
 @pytest.mark.unit
 async def test_verify_news_items_unexpected_exception_keeps_all(mocker):
     """A non-HTTP exception from get_items must keep all results (fail open).
@@ -505,6 +544,23 @@ async def test_verify_files_transient_5xx_keeps(mocker):
     assert result == {7}
 
 
+@pytest.mark.unit
+async def test_verify_files_429_keeps_as_transient(mocker):
+    """HTTP 429 from get_file_info must NOT silently drop file results."""
+    webdav_client = SimpleNamespace(
+        get_file_info=mocker.AsyncMock(side_effect=_http_error(429))
+    )
+    client = SimpleNamespace(webdav=webdav_client, username="alice")
+
+    result = await _verify_files(
+        client,
+        [_make_result(7, doc_type="file", metadata={"path": "x.txt"})],
+        _sem(),
+    )
+
+    assert result == {7}
+
+
 @pytest.mark.unit
 async def test_verify_files_unexpected_exception_keeps(mocker):
     """A non-HTTP exception from get_file_info must not drop the result.
@@ -623,6 +679,29 @@ async def test_verify_deck_cards_transient_5xx_keeps(mocker):
     assert result == {42}
 
 
+@pytest.mark.unit
+async def test_verify_deck_cards_429_keeps_as_transient(mocker):
+    """HTTP 429 from get_card must NOT silently shrink result pages."""
+    deck_client = SimpleNamespace(
+        get_card=mocker.AsyncMock(side_effect=_http_error(429))
+    )
+    client = SimpleNamespace(deck=deck_client, username="alice")
+
+    result = await _verify_deck_cards(
+        client,
+        [
+            _make_result(
+                42,
+                doc_type="deck_card",
+                metadata={"board_id": 1, "stack_id": 2},
+            )
+        ],
+        _sem(),
+    )
+
+    assert result == {42}
+
+
 @pytest.mark.unit
 async def test_verify_deck_cards_unexpected_exception_keeps(mocker):
     """Non-HTTP exception from get_card → fail-open, keep result."""