feat(stats): added tool usage dashboard
- Parsed assistant `toolCall` blocks and `toolResult` messages into persisted `tool_calls` rows with one-shot historical backfill.
- Added tool aggregate queries, model breakdowns, call time-series data, and the `/api/stats/tools` dashboard endpoint.
- Added the `/#/tools` route with summary metrics, stacked calls-over-time chart, per-tool table, and model breakdown panel.
- Added end-to-end stats coverage for tool ingestion, result/error linkage, fork deduplication, incremental updates, and dashboard shaping.
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