fix: 3 bugs from system logs — timedelta, scoring IndexError, lxml

- database.py: add timedelta to datetime import (used in
  get_recent_economic_surprises, was raising NameError)
- ai_analyzer.py: scoring split was searching French string
  'Retourne UNIQUEMENT ce JSON valide:' but prompt is now in English
  'Return ONLY this valid JSON:' — caused IndexError crashing every cycle
- requirements.txt: add lxml>=5.0.0 (yfinance earnings_dates dependency,
  was silently failing all 23 ticker fetches every hour)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/database.py

@@ -4,7 +4,7 @@ SQLite persistence layer for portfolio positions, custom patterns, and config.
 import sqlite3
 import json
 import os
-from datetime import datetime
+from datetime import datetime, timedelta
 from typing import List, Dict, Any, Optional
 
 DB_PATH = os.path.join(os.path.dirname(__file__), "..", "data", "geooptions.db")