feat: Add Scan History tab with rescan capability

- New Scan History tab shows all past scans
- Each scan shows: tool, target, status, timestamp, findings count
- Rescan button to quickly re-run any previous scan with same settings
- Copy command button copies scan command to clipboard and terminal
- Clear All button to purge scan history
- Relative timestamps (e.g. '5 min ago', '2 hours ago')
- Status badges: running (yellow), completed (green), failed (red)
- Backend endpoints for clearing scan history

services/hackgpt-api/app/main.py

@@ -721,6 +721,14 @@ async def list_scans():
     return list(scan_results.values())
 
 
+@app.delete("/scans/clear")
+async def clear_scans():
+    """Clear all scan history."""
+    global scan_results
+    scan_results = {}
+    return {"status": "cleared", "message": "All scan history cleared"}
+
+
 # ============== Output Parsing ==============
 
 def parse_tool_output(tool: str, output: str) -> Dict[str, Any]: