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/semantic.py

@@ -54,6 +54,7 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
         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 semantic search using vector similarity.
@@ -78,6 +79,8 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
                 seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
             modified_before: Inclusive upper bound on ``modified_at`` (Unix
                 seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
+            path_prefix: Folder/path filter on ``file_path`` (files only);
+                ``None`` ⇒ no path filter (ADR-027 Phase 2).
             **kwargs:
                 - score_threshold (float): override the instance default
 
@@ -118,6 +121,7 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
             doc_type=doc_type,
             modified_after=modified_after,
             modified_before=modified_before,
+            path_prefix=path_prefix,
         )
 
         # ACL pre-filter (design §11), opt-in via ACL_PREFILTER_ENABLED and OFF