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/vector/qdrant_client.py

@@ -62,6 +62,14 @@ _PAYLOAD_INDEX_FIELDS: dict[str, PayloadSchemaType] = {
     # is idempotent, so existing collections gain this index at startup with no
     # content re-index and no operator action.
     "modified_at": PayloadSchemaType.INTEGER,
+    # file_path is the ADR-027 Phase 2 path filter field: searches apply
+    # MatchText(key="file_path", text=path_prefix) (see
+    # search/access_filter.build_base_filter_conditions). MatchText needs a TEXT
+    # index on server Qdrant; the value is already on every file point
+    # (processor.py, doc_type == "file" only), so this is a no-content-re-index
+    # migration like modified_at. Local/embedded qdrant-client matches by
+    # substring without an index, so dev stacks work without it too.
+    "file_path": PayloadSchemaType.TEXT,
 }
 
 # Sentinel point that records "this collection has been backfilled to str