feat: add ranking table sync and API endpoints

Sync Ranking table from SQL Server to PostgreSQL with ranking_ca,
ranking_qte, ranking_mag_* metrics. Expose via /api/ranking and
integrate into /api/articles/:id/referentiel response.

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

sync/index.js

@@ -7,6 +7,7 @@ const { syncStock }        = require('./tables/stock');
 const { syncMouvements }   = require('./tables/mouvements');
 const { syncCommandes }    = require('./tables/commandes');
 const { syncReferentiel }  = require('./tables/referentiel');
+const { syncRanking }      = require('./tables/ranking');
 const { closeAll }         = require('./db');
 const {
   startDashboard, registerSyncFn,
@@ -24,6 +25,7 @@ const STEPS = [
   { name: 'stock',        fn: syncStock },
   { name: 'mouvements',   fn: syncMouvements },
   { name: 'commandes',    fn: syncCommandes },
+  { name: 'ranking',      fn: syncRanking },
 ];
 
 async function syncAll(force = false) {