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/admin.py

@@ -0,0 +1,24 @@
+from django.contrib import admin
+from .models import AdSurface, AdPlacement
+
+
+@admin.register(AdSurface)
+class AdSurfaceAdmin(admin.ModelAdmin):
+    list_display = ('key', 'name', 'max_slots', 'layout_type', 'sort_behavior', 'is_active', 'active_count')
+    list_filter = ('is_active', 'layout_type')
+    search_fields = ('key', 'name')
+    readonly_fields = ('created_at',)
+
+
+@admin.register(AdPlacement)
+class AdPlacementAdmin(admin.ModelAdmin):
+    list_display = (
+        'id', 'event', 'surface', 'status', 'scope', 'priority',
+        'rank', 'boost_label', 'start_at', 'end_at', 'created_at',
+    )
+    list_filter = ('status', 'scope', 'priority', 'surface')
+    list_editable = ('status', 'scope', 'rank')
+    search_fields = ('event__name', 'event__title', 'boost_label')
+    raw_id_fields = ('event', 'created_by', 'updated_by')
+    readonly_fields = ('created_at', 'updated_at')
+    ordering = ('surface', 'rank')