Add Steam library import via OpenID

- Steam OpenID flow: user authenticates with Steam, we get their Steam ID
- Server-side API key fetches their owned games with playtime
- Import page shows full library, marks already-imported games
- Imported games land in backlog as GAMES items with hours_played set
- STEAM_API_KEY env var plumbed into both prod and dev containers

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

backlogger/urls.py

@@ -7,4 +7,7 @@ urlpatterns = [
     path('add/', views.item_add, name='add'),
     path('<int:pk>/edit/', views.item_edit, name='edit'),
     path('<int:pk>/delete/', views.item_delete, name='delete'),
+    path('steam/login/', views.steam_login, name='steam_login'),
+    path('steam/callback/', views.steam_callback, name='steam_callback'),
+    path('steam/import/', views.steam_import, name='steam_import'),
 ]