refactor(constants): initialize LEAD_TEMPLATE_MAP as an empty dictionary

refactor(page): clean up import statements for better readability
fix(tests): update assertions for clarity and consistency

backend/tests/test_template_size_budget.py

@@ -11,9 +11,7 @@ TEMPLATES_DIR = Path(__file__).resolve().parents[1] / "templates"
 
 def test_heartbeat_templates_fit_in_injected_context_limit() -> None:
     """Heartbeat templates must stay under gateway injected-context truncation limit."""
-    targets = (
-        "BOARD_HEARTBEAT.md.j2",
-    )
+    targets = ("BOARD_HEARTBEAT.md.j2",)
     for name in targets:
         size = (TEMPLATES_DIR / name).stat().st_size
         assert (