feat(economic): WoW price tracking + weekly cadence for BigMac & Grocery panels (#1974)

* feat(economic): add WoW tracking and fix plumbing for bigmac/grocery-basket panels

Phase 1 — Fix Plumbing:
- Adjust CACHE_TTL to 10 days (864000s) for bigmac and grocery-basket seeds
- Align health.js SEED_META maxStaleMin to 10080 (7 days) for both
- Add grocery-basket and bigmac to seed-health.js SEED_DOMAINS with intervalMin: 5040
- Refactor publish.ts writeSnapshot to accept advanceSeedMeta param; only
  advance seed-meta when fresh data exists (overallFreshnessMin < 120)
- Add manual-fallback-only comment to seed-consumer-prices.mjs

Phase 2 — Week-over-Week Tracking:
- Add wow_pct field to BigMacCountryPrice and CountryBasket proto messages
- Add wow_avg_pct, wow_available, prev_fetched_at to both response protos
- Regenerate client/server TypeScript from updated protos
- Add readCurrentSnapshot() helper + WoW computation to seed-bigmac.mjs
  and seed-grocery-basket.mjs; write :prev key via extraKeys
- Update BigMacPanel.ts to show per-country WoW column and global avg summary
- Update GroceryBasketPanel.ts to show WoW badge on total row and basket avg summary
- Add .bm-wow-up, .bm-wow-down, .bm-wow-summary, .gb-wow CSS classes
- Fix server handlers to include new WoW fields in fallback responses

* fix(economic): guard :prev extraKey against null on first seed run; eliminate double freshness query in publish.ts

* refactor(economic): address code review findings from PR #1974

- Extract readSeedSnapshot() into _seed-utils.mjs (DRY: was duplicated
  verbatim in seed-bigmac and seed-grocery-basket)
- Add FRESH_DATA_THRESHOLD_MIN constant in publish.ts (replace magic 120)
- Fix seed-consumer-prices.mjs contradictory JSDoc (remove stale
  "Deployed as: Railway cron service" line that contradicted manual-only warning)
- Add i18n keys panels.bigmacWow / panels.bigmacCountry to en.json
- Replace hardcoded "WoW" / "Country" with t() calls in BigMacPanel
- Replace IIFE-in-ternary pattern with plain if blocks in BigMacPanel
  and GroceryBasketPanel (P2/P3 from code review)

* fix(publish): gate advanceSeedMeta on any-retailer freshness, not average

overallFreshnessMin is the arithmetic mean across all retailers, so with
1 fresh + 2 stale retailers the average can exceed 120 min and suppress
seed-meta advancement even while fresh data is being published.

Use retailers.some(r => r.freshnessMin < 120) to correctly implement
"at least one retailer scraped within the last 2 hours."

docs/api/EconomicService.openapi.yaml

@@ -981,6 +981,13 @@ components:
                     type: string
                 upstreamUnavailable:
                     type: boolean
+                wowAvgPct:
+                    type: number
+                    format: double
+                wowAvailable:
+                    type: boolean
+                prevFetchedAt:
+                    type: string
         CountryBasket:
             type: object
             properties:
@@ -1002,6 +1009,9 @@ components:
                     type: array
                     items:
                         $ref: '#/components/schemas/GroceryItemPrice'
+                wowPct:
+                    type: number
+                    format: double
         GroceryItemPrice:
             type: object
             properties:
@@ -1038,6 +1048,13 @@ components:
                     type: string
                 mostExpensiveCountry:
                     type: string
+                wowAvgPct:
+                    type: number
+                    format: double
+                wowAvailable:
+                    type: boolean
+                prevFetchedAt:
+                    type: string
         BigMacCountryPrice:
             type: object
             properties:
@@ -1062,6 +1079,9 @@ components:
                     type: string
                 available:
                     type: boolean
+                wowPct:
+                    type: number
+                    format: double
         GetNationalDebtRequest:
             type: object
             description: GetNationalDebtRequest requests national debt data for all countries.