fix: mvtart sync ON CONFLICT DO NOTHING + doc ranking API

- Add ON CONFLICT DO NOTHING to mvtart insert to work with unique index
- Document ranking endpoints in API_DOCUMENTATION.md

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

sync/tables/mouvements.js

@@ -25,7 +25,7 @@ async function insertChunk(pg, table, rows, cols) {
     return `(${ph.join(', ')})`;
   });
   await pg.query(
-    `INSERT INTO ${table} (${cols.join(', ')}) VALUES ${placeholders.join(', ')}`,
+    `INSERT INTO ${table} (${cols.join(', ')}) VALUES ${placeholders.join(', ')} ON CONFLICT DO NOTHING`,
     values
   );
 }