feat: restructure pages under trips/japan-korea-2026 entity

- Create trips/japan-korea-2026/{tracker,map,stats,stories} hierarchy
- Move 8 entry folders from 01.tracker into trips/.../01.tracker/
- Add active_trip: japan-korea-2026 to site.yaml
- Whitelist GPX file type in media.yaml

pages/01.trips/japan-korea-2026/04.stories/stories.md

@@ -0,0 +1,5 @@
+---
+title: Stories
+template: stories
+published: true
+---

pages/01.trips/japan-korea-2026/trip.md

@@ -0,0 +1,10 @@
+---
+title: 'Japan & Korea 2026'
+template: trip
+date: '2026-06-17'
+date_start: '2026-06-17'
+date_end: ''
+cover_image: ''
+content:
+    items: '@self.children'
+---

pages/01.trips/trips.md

@@ -0,0 +1,9 @@
+---
+title: Trips
+template: trips
+content:
+    items: '@self.children'
+    order:
+        by: date
+        dir: desc
+---