feat(ui,api,admin): sveltekit admin spa, usage by day/agent/conversation/model, rate limits, memory tree, turn snapshot

src/beaver_gateway/core/turn_capture.py

@@ -24,6 +24,8 @@ class TurnUsage:
     cost_usd: float | None = None
     duration_ms: int | None = None
     num_turns: int | None = None
+    model_usage: dict[str, Any] | None = None
+    """``ResultMessage.model_usage`` verbatim: per-model tokens, cost, web searches."""
 
 
 @dataclass