feat: add backtesting functionality with UI and API endpoints

- Introduced backtesting page and fragment in the dashboard for running backtests and viewing recent reports.
- Implemented backtest run logic with configuration options including event path, starting balances, trade capital, and fee profiles.
- Added recent backtest reports storage and retrieval.
- Created a new strategy module for statistical arbitrage experiments with validation on configuration parameters.
- Updated settings to include parameters for the statistical arbitrage strategy.
- Enhanced dashboard controls to support the new strategy mode.
- Added unit tests for backtesting functionality and strategy validation.
- Updated templates for backtesting UI integration.

docs/architecture/current-implementation.md

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+# Current Implementation Snapshot
+
+This document summarizes the code that exists now, not the original plan.
+
+## Runtime
+
+- FastAPI app starts from [src/arbitrade/api/app.py](../../src/arbitrade/api/app.py).
+- Settings come from `pydantic-settings` in [src/arbitrade/config/settings.py](../../src/arbitrade/config/settings.py).
+- DuckDB is initialized and migrated on startup.
+- Runtime recovery persists and restores control state and snapshots.
+
+## Market Data and Detection
+
+- Kraken market data is handled by native WS and thin REST code.
+- Incremental triangular detector is implemented in [src/arbitrade/detection/engine.py](../../src/arbitrade/detection/engine.py).
+- Currency graph and cycle indexing live in [src/arbitrade/detection/graph.py](../../src/arbitrade/detection/graph.py).
+
+## Execution and Risk
+
+- Multi-leg execution sequencer exists for triangular cycles.
+- Pre-trade validation and trade-limit guards are wired into execution flow.
+- Kill switch and stop conditions are supported.
+
+## Dashboard
+
+- Server-rendered dashboard uses FastAPI + Jinja2 + HTMX.
+- Live metrics, overview, controls, charts, and audit fragments are exposed as separate endpoints.
+- Dedicated backtesting page exists at `/dashboard/backtesting`.
+
+## Backtesting
+
+- Replay engine lives in [src/arbitrade/backtesting/replay.py](../../src/arbitrade/backtesting/replay.py).
+- Parameter sweep runner lives in [src/arbitrade/backtesting/sweep.py](../../src/arbitrade/backtesting/sweep.py).
+- Backtesting UI runs replay from a JSONL file, stores recent reports in app state, and exposes a recent-reports API.
+- Experimental stat-arb scaffold lives in [src/arbitrade/strategy/stat_arb.py](../../src/arbitrade/strategy/stat_arb.py) and is gated by feature flag.
+
+## Deployment
+
+- Dockerfile installs runtime dependencies from `requirements/latest-runtime.in`.
+- CI publishes `git.allucanget.biz/allucanget/arbitrade:latest`.
+- Coolify deploys the prebuilt image and owns runtime env vars and persistent storage.
+
+## Current Gaps
+
+- Cross-exchange arbitrage remains deferred.
+- Stat-arb is experimental, not part of default live strategy.
+- Backtesting UI is functional but still a single-run/report workflow, not a full job queue.