feat(trade): add effective tariff rate data source (beyond WTO MFN baseline)

[koala73]

Problem

The Trade Policy panel currently shows WTO MFN (Most Favored Nation) applied tariff rates, which are the baseline rates countries commit to at the WTO level. The US MFN rate has been ~3.3% for a decade and does not reflect:

• Section 301 tariffs on China (currently 60%+)
• Section 232 tariffs on steel/aluminum (25%)
• Reciprocal tariffs (10-50% depending on country)
• Executive order tariffs

Users see "US tariff rate: 3.3%" and cannot understand why everyone is talking about high tariffs. The Revenue tab (308% YoY customs revenue increase) tells the impact story, but the Restrictions and Tariffs tabs show misleading low numbers.

Proposed solution

Add a new data source for effective/applied tariff rates that captures unilateral tariff actions. Potential sources (in order of feasibility):

1. Yale Budget Lab (budgetlab.yale.edu) - publishes effective US tariff rate tracker, currently ~22-28% weighted average. May have scrapeable data or API.
2. Peterson Institute (piie.com) - publishes tariff trackers with country-level breakdowns. Research publications, likely needs scraping.
3. Tax Foundation - publishes tariff impact analyses with effective rate estimates.
4. USITC Harmonized Tariff Schedule - authoritative but complex (thousands of line items per country).
5. US Census Bureau FT900 - trade deficit data that shows volume impact.

Implementation ideas

• New tab "Effective Rates" or integrate into existing Restrictions tab
• Show WTO MFN rate alongside actual effective rate (the gap IS the story)
• Highlight countries with largest gap between MFN and effective rate
• Could correlate with Revenue tab data (higher effective rate = higher revenue)

Context

• Trade Policy panel: src/components/TradePolicyPanel.ts
• Trade service: src/services/trade/index.ts
• RPC handlers: server/worldmonitor/trade/v1/
• Seed script: scripts/seed-supply-chain-trade.mjs
• Revenue tab already shows the impact (FY2026 YTD $144.3B vs FY2025 $35.4B, +308%)

[FayezBast]

I would like to work on it