refactor(search): structural per-instance query side-channel; doc search billing gap

Round-7 claude-review (no blockers):

- 🟡 query_token_count/query_embedding were class-level defaults on
  SearchAlgorithm, relying on each subclass's __init__ to shadow them. Added
  SearchAlgorithm.__init__ that sets both as instance attributes and had
  BM25HybridSearchAlgorithm + SemanticSearchAlgorithm call super().__init__(),
  so per-request concurrency isolation is structural, not by convention.
- 🟡 Documented the v1 search-path billing gap: record_search_usage fires only
  on a fully successful search, so if the query embed succeeded (provider billed
  + Prometheus recorded) but a later step (Qdrant/verify) raised, no
  tokens_embedded billing row is written. Added a NOTE at the call site.

Left as-is (reasons in PR reply): deployment sequencing (CP METRIC_EVENT_NAMES
already renamed; pipeline inert); Ollama _detect_dimension double dimension-set
(idempotent, same value); SonarQube issues — 1 is the deliberate TODO(#282)
(INFO), 4 are S7503 false positives on async test stubs that must be awaitable
(gate green).

Deck #284.

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

nextcloud_mcp_server/search/algorithms.py

@@ -292,9 +292,19 @@ class SearchAlgorithm(ABC):
             per-request, so this side-channel is concurrency-safe.
     """
 
+    # Class-level defaults are a safety net; __init__ shadows them per instance.
     query_embedding: list[float] | None = None
     query_token_count: int | None = None
 
+    def __init__(self) -> None:
+        # Set the query-embedding side-channel as instance attributes so
+        # concurrent SearchAlgorithm instances never share it through the
+        # class-level defaults above — per-request isolation by construction,
+        # not just by the convention that each subclass redeclares them.
+        # Subclasses with their own __init__ should call super().__init__().
+        self.query_embedding: list[float] | None = None
+        self.query_token_count: int | None = None
+
     @abstractmethod
     async def search(
         self,