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/api/visualization.py

@@ -225,6 +225,8 @@ async def unified_search(request: Request) -> JSONResponse:
         include_pca = body.get("include_pca", False)
         include_chunks = body.get("include_chunks", True)
         doc_types = body.get("doc_types")  # Optional filter
+        # ADR-027 Phase 2 path filter (files only); blank ⇒ no filter.
+        path_prefix = (body.get("path_prefix") or "").strip() or None
 
         if not query:
             return JSONResponse({"results": [], "total_found": 0})
@@ -266,6 +268,7 @@ async def unified_search(request: Request) -> JSONResponse:
                                 accessible_owners=owners,
                                 modified_after=modified_after,
                                 modified_before=modified_before,
+                                path_prefix=path_prefix,
                             )
                         )
                 # Sort, then cap to a fixed over-fetch budget before the result
@@ -285,6 +288,7 @@ async def unified_search(request: Request) -> JSONResponse:
                     accessible_owners=owners,
                     modified_after=modified_after,
                     modified_before=modified_before,
+                    path_prefix=path_prefix,
                 )
             return results
 
@@ -436,6 +440,8 @@ async def vector_search(request: Request) -> JSONResponse:
         limit = min(body.get("limit", 10), 50)  # Enforce max limit
         include_pca = body.get("include_pca", True)
         doc_types = body.get("doc_types")  # Optional list of document types
+        # ADR-027 Phase 2 path filter (files only); blank ⇒ no filter.
+        path_prefix = (body.get("path_prefix") or "").strip() or None
         # ADR-027 modified-date range filter. Accepts RFC 3339 / ISO 8601
         # datetimes or Unix seconds; normalized to int Unix seconds. None ⇒ open.
         try:
@@ -501,6 +507,7 @@ async def vector_search(request: Request) -> JSONResponse:
                                 accessible_owners=owners,
                                 modified_after=modified_after,
                                 modified_before=modified_before,
+                                path_prefix=path_prefix,
                             )
                         )
                 # Sort merged results by score and limit
@@ -515,6 +522,7 @@ async def vector_search(request: Request) -> JSONResponse:
                     accessible_owners=owners,
                     modified_after=modified_after,
                     modified_before=modified_before,
+                    path_prefix=path_prefix,
                 )
             return results