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/requirements.txt

@@ -18,3 +18,4 @@ python-multipart==0.0.12
 apscheduler==3.10.4
 pytz==2024.2
 ta==0.11.0
+lxml>=5.0.0