feat(news): replace keyword sentiment with VADER

vaderSentiment==3.3.2 added to requirements.txt.

_score_headlines now:
- scores each article (title + description) with VADER compound ∈ [-1, +1]
- filters out articles with |compound| ≤ 0.05 (no clear signal)
- weights remaining articles by recency (GNews newest-first, rank 0 → highest weight)
- returns weighted mean clamped to [-1, +1]

Removes the custom keyword sets (_POSITIVE/_NEGATIVE) and the set-based
bag-of-words algorithm that capped scores at ~±0.5 in practice.

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

requirements.txt

@@ -10,6 +10,7 @@ pydantic==2.7.0
 
 # Utils
 python-dotenv==1.0.1
+vaderSentiment==3.3.2
 
 # Testing
 pytest==8.2.0