feat(ad_control): new AdSurface + AdPlacement module for placement-based featured/top events

- New ad_control Django app: AdSurface + AdPlacement models with GLOBAL/LOCAL scope
- Admin CRUD API at /api/v1/ad-control/ (JWT-protected): surfaces, placements, picker events
- Placement lifecycle: DRAFT → ACTIVE|SCHEDULED → EXPIRED|DISABLED
- LOCAL scope: Haversine ≤ 50km from event lat/lng (fixed radius, no config needed)
- Consumer APIs: /api/events/featured-events/ and /api/events/top-events/ rewritten
  to use placement-based queries (same URL paths + response shape — no breaking changes)
- Seed command: seed_surfaces --migrate converts existing is_featured/is_top_event booleans
- mount: admin_api/urls.py → ad-control/, mobile_api/urls.py → replaced consumer views
- settings.py: added ad_control to INSTALLED_APPS

ad_control/apps.py

@@ -0,0 +1,7 @@
+from django.apps import AppConfig
+
+
+class AdControlConfig(AppConfig):
+    default_auto_field = 'django.db.models.BigAutoField'
+    name = 'ad_control'
+    verbose_name = 'Ad Control'