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>

tests/unit/vector/test_qdrant_client.py

@@ -88,6 +88,13 @@ def test_modified_at_indexed_as_integer():
     assert _PAYLOAD_INDEX_FIELDS.get("modified_at") == PayloadSchemaType.INTEGER
 
 
+@pytest.mark.unit
+def test_file_path_indexed_as_text():
+    """ADR-027 Phase 2: the path filter uses MatchText, which needs a TEXT index
+    on file_path (server Qdrant); local qdrant-client matches by substring."""
+    assert _PAYLOAD_INDEX_FIELDS.get("file_path") == PayloadSchemaType.TEXT
+
+
 # ---------------------------------------------------------------------------
 # _ensure_payload_indexes
 # ---------------------------------------------------------------------------