feat: effective tariff rate source (#1790)

* feat: effective tariff rate source

* fix(trade): extract parse helpers, fix tests, add health monitoring

- Extract htmlToPlainText/toIsoDate/parseBudgetLabEffectiveTariffHtml
  to scripts/_trade-parse-utils.mjs so tests can import directly
- Fix toIsoDate to use month-name lookup instead of fragile
  new Date(\`\${text} UTC\`) which is not spec-guaranteed
- Replace new Function() test reconstruction with direct ESM import
- Add test fixtures for parser patterns 2 and 3 (previously untested)
- Add tariffTrendsUs to health.js STANDALONE_KEYS + SEED_META
  (key trade:tariffs:v1:840:all:10, maxStaleMin 900 = 2.5x the 6h TTL)

* fix(test): update sourceVersion assertion for budgetlab addition

---------

Co-authored-by: Elie Habib <elie.habib@gmail.com>

docs/api/TradeService.openapi.yaml

@@ -334,6 +334,8 @@ components:
                 upstreamUnavailable:
                     type: boolean
                     description: True if upstream fetch failed and results may be stale/empty.
+                effectiveTariffRate:
+                    $ref: '#/components/schemas/EffectiveTariffRate'
             description: Response containing tariff trend datapoints.
         TariffDataPoint:
             type: object
@@ -363,6 +365,26 @@ components:
                     type: string
                     description: WTO indicator code used for this datapoint.
             description: Single tariff data point for a reporter-partner-product combination.
+        EffectiveTariffRate:
+            type: object
+            properties:
+                sourceName:
+                    type: string
+                    description: Source name for the effective-rate estimate.
+                sourceUrl:
+                    type: string
+                    description: Canonical source URL for the estimate/methodology.
+                observationPeriod:
+                    type: string
+                    description: Human-readable observation period (for example "December 2025").
+                updatedAt:
+                    type: string
+                    description: ISO 8601 date when the source page was last updated, if known.
+                tariffRate:
+                    type: number
+                    format: double
+                    description: Effective tariff rate (percentage).
+            description: Current effective tariff estimate for countries with coverage beyond WTO MFN baselines.
         GetTradeFlowsRequest:
             type: object
             properties: