feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Adds Exa as a new search source in the Research News feed, providing AI-powered neural/semantic web search alongside arXiv, HuggingFace, X, and Xiaohongshu
• New search_exa.py script using the Exa REST API with configurable search queries, category filters (research paper, news, company, etc.), and date ranges
• Full frontend integration: source icon, filter bar button, settings dialog with query/category/date controls, dashboard stat card, unified feed styling, and i18n support (en, zh-CN, ko)
• Uses shared scoring_utils for consistent relevance/recency/popularity/quality scoring across all sources

Usage

# Set your Exa API key
export EXA_API_KEY=your-exa-api-key

# The Exa source appears automatically in the Research News dashboard
# Configure search queries, category, and date range in Settings

Files changed

• server/scripts/research-news/search_exa.py — new Exa search script
• server/routes/news.js — register Exa in SOURCE_REGISTRY
• src/components/news-dashboard/view/useNewsDashboardData.ts — add exa to NewsSourceKey type
• src/components/news-dashboard/view/SourceFilterBar.tsx — add Exa button/colors
• src/components/news-dashboard/view/SourceIcon.tsx — add Exa icon
• src/components/news-dashboard/view/SourceSettingsDialog.tsx — add Exa-specific settings UI
• src/components/news-dashboard/view/NewsDashboard.tsx — add Exa stat card
• src/components/news-dashboard/view/UnifiedFeed.tsx — add Exa feed styling
• src/i18n/locales/{en,zh-CN,ko}/news.json — add Exa translations
• public/icons/news/exa.svg — Exa source icon
• .env.example — add EXA_API_KEY

Test plan

• Set EXA_API_KEY env var and start the server
• Navigate to Research News dashboard and verify Exa tab appears
• Click Exa tab, open Settings, configure queries and category
• Click "Start Search" and verify results appear with correct scoring
• Verify settings persist across page reloads
• Verify all three languages render correctly (en, zh-CN, ko)
• Verify existing sources (arXiv, HuggingFace, X, Xiaohongshu) still work correctly