Implement Backtest Engine - Historical Data Replay

Implements issue #7 - Backtest Engine for historical strategy testing.

Changes:
- Created AveCloudClient for fetching klines from AVE Cloud Data API
- Implemented BacktestEngine with condition matching (price_drop, price_rise, volume_spike, price_level)
- Implemented signal generation and portfolio simulation
- Calculates metrics: total_return, win_rate, max_drawdown, sharpe_ratio, total_trades
- Implemented async/background backtest execution via FastAPI BackgroundTasks
- Stores results in backtests table and signals table
- All backtest API endpoints with JWT auth and ownership validation

API Endpoints:
- POST /api/bots/{id}/backtest - Start backtest
- GET /api/bots/{id}/backtest/{run_id} - Get status/results
- GET /api/bots/{id}/backtests - List all backtests
- POST /api/bots/{id}/backtest/{run_id}/stop - Stop running backtest

src/backend/app/services/backtest/ave_client.py

@@ -0,0 +1,70 @@
+import httpx
+from typing import List, Dict, Any, Optional
+from datetime import datetime
+
+
+class AveCloudClient:
+    BASE_URL = "https://prod.ave-api.com"
+
+    def __init__(self, api_key: str, plan: str = "free"):
+        self.api_key = api_key
+        self.plan = plan
+
+    def _headers(self) -> Dict[str, str]:
+        return {"X-API-KEY": self.api_key}
+
+    async def get_klines(
+        self,
+        token_id: str,
+        interval: str = "1h",
+        limit: int = 100,
+        start_time: Optional[int] = None,
+        end_time: Optional[int] = None,
+    ) -> List[Dict[str, Any]]:
+        url = f"{self.BASE_URL}/v2/klines/token/{token_id}"
+        params = {"interval": interval, "limit": limit}
+        if start_time:
+            params["start_time"] = start_time
+        if end_time:
+            params["end_time"] = end_time
+
+        async with httpx.AsyncClient() as client:
+            response = await client.get(
+                url, headers=self._headers(), params=params, timeout=30.0
+            )
+            response.raise_for_status()
+            data = response.json()
+            if data.get("status") == 200:
+                return data.get("data", [])
+            raise Exception(f"Failed to fetch klines: {data}")
+
+    async def get_token_price(self, token_id: str) -> Optional[Dict[str, Any]]:
+        url = f"{self.BASE_URL}/v2/tokens/price"
+        async with httpx.AsyncClient() as client:
+            response = await client.post(
+                url,
+                headers=self._headers(),
+                json={"token_ids": [token_id]},
+                timeout=30.0,
+            )
+            response.raise_for_status()
+            data = response.json()
+            if data.get("status") == 200:
+                prices = data.get("data", {})
+                return prices.get(token_id)
+            return None
+
+    async def get_batch_prices(self, token_ids: List[str]) -> Dict[str, Dict[str, Any]]:
+        url = f"{self.BASE_URL}/v2/tokens/price"
+        async with httpx.AsyncClient() as client:
+            response = await client.post(
+                url,
+                headers=self._headers(),
+                json={"token_ids": token_ids},
+                timeout=30.0,
+            )
+            response.raise_for_status()
+            data = response.json()
+            if data.get("status") == 200:
+                return data.get("data", {})
+            return {}