feat(search): ADR-027 Phase 2 — file-path filter

Add a path_prefix filter to semantic search, honoured on both the MCP tool and
the dense-only visualization/API paths through the shared filter contract.

- build_base_filter_conditions: append FieldCondition(file_path,
  MatchText(path_prefix)) when set. file_path is only on doc_type == "file"
  points, so a non-empty path_prefix implicitly restricts to files.
- Promote path_prefix to an explicit keyword param on the SearchAlgorithm ABC
  and both algorithms; thread it through nc_semantic_search (blank ⇒ no filter),
  the /api/v1 search endpoints, and the viz route.
- Add a file_path TEXT payload index to _PAYLOAD_INDEX_FIELDS (no content
  re-index; idempotent startup migration). MatchText tokenizes on server Qdrant
  and matches by substring on local/embedded qdrant-client — both serve folder
  scoping.
- Update ADR-027 (Phase 2 implemented; readiness table; semantics note). Tests.

Refs ADR-027 Phase 2. Deck #177.

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

nextcloud_mcp_server/search/algorithms.py

@@ -291,6 +291,7 @@ class SearchAlgorithm(ABC):
         accessible_owners: list[str] | None = None,
         modified_after: int | None = None,
         modified_before: int | None = None,
+        path_prefix: str | None = None,
         **kwargs: Any,
     ) -> list[SearchResult]:
         """Execute search with the given parameters.
@@ -312,6 +313,10 @@ class SearchAlgorithm(ABC):
                 ``accessible_owners`` (ADR-027). ``None`` ⇒ open-ended.
             modified_before: Optional inclusive upper bound on ``modified_at``
                 (Unix seconds, UTC). ``None`` ⇒ open-ended.
+            path_prefix: Optional folder/path filter on the ``file_path`` payload
+                field (ADR-027 Phase 2). Only ``doc_type == "file"`` points carry
+                ``file_path``, so a non-empty value implicitly restricts results
+                to files. ``None`` ⇒ no path filter.
             **kwargs: Algorithm-specific parameters
 
         Returns: