chore(canvas): drop unused imports flagged by ruff

calliope-backend/tests/test_canvas_db.py

@@ -1,8 +1,6 @@
 """Canvas v2.1 Phase 1 — canvas/canvas_node tables, scoping, and cascades."""
 from __future__ import annotations
 
-import sqlite3
-
 
 def _get_canvas(client, canvas_id: int):
     r = client.get(f"/api/canvas/{canvas_id}")

calliope-backend/tests/test_canvas_router.py

@@ -1,8 +1,6 @@
 """Canvas v2.1 Phase 1 — CRUD router, get-or-create scoping, entity auto-seed."""
 from __future__ import annotations
 
-import json as _json
-
 import calliope.config as config_module
 from calliope.db import get_db