chore(slideshow): renumber migrations to 138/139 (after whatsapp #649's 137)

PR #649 takes migration 137 (add_whatsapp_template_language). Renumber the
slideshow migrations to slot in after it:
- 137_add_slideshow_share.js  -> 138_add_slideshow_share.js
- 138_add_slideshow_styling.js -> 139_add_slideshow_styling.js
and update the slideshow migration-number references in comments/types. No
content change — both are additive + addColumnIfNotExists-guarded, so re-running
under the new filename on an already-migrated DB is a safe no-op.

backend/migrations/core/139_add_slideshow_styling.js

@@ -1,6 +1,6 @@
 const { addColumnIfNotExists } = require('../helpers');
 
-// Live Slideshow styling (migration 137 follow-up):
+// Live Slideshow styling (migration 138 follow-up):
 //   - a ZDF/ARD-ident-style watermark: a white, semi-transparent logo in a
 //     corner of the projected slideshow (sourced from the site branding logo
 //     or the event's own logo).
@@ -54,5 +54,5 @@ exports.up = async function up(knex) {
 };
 
 exports.down = async function down() {
-  // Safe rollback - intentionally no-op to avoid data loss (mirrors 074/137).
+  // Safe rollback - intentionally no-op to avoid data loss (mirrors 074/138).
 };