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>

nextcloud_mcp_server/server/semantic.py

@@ -156,9 +156,20 @@ def configure_semantic_tools(mcp: FastMCP):
                 )
                 all_results.extend(unverified_results)
             else:
-                # Search specific document types
-                # For each requested type, execute search and combine results
-                # under the same 2× over-fetch budget (see NOTE above).
+                # Search specific document types.
+                #
+                # Per-Qdrant-query cost: this branch issues ONE query per
+                # requested doc_type, each capped at `limit * 2`. With N
+                # types in `doc_types`, the pre-merge result pool is
+                # therefore N × `limit * 2`, NOT `limit * 2`. That is more
+                # Qdrant work than the cross-app branch above (which makes a
+                # single multi-type query returning `limit * 2` total).
+                #
+                # The post-merge trim below clamps the pool back down to
+                # `limit * 2` so verification (and the Nextcloud round-trips
+                # it triggers) sees the same budget as the cross-app branch.
+                # The per-type Qdrant cost remains higher; pre-trim cost
+                # scales linearly with len(doc_types).
                 for dtype in doc_types:
                     unverified_results = await search_algo.search(
                         query=query,