Add inbound country flags and node load in location picker

- Inbound.CountryCode (ISO alpha-2), set on publish; flag shown before
  the location name (SVG via country-flag-icons on the site, emoji in bot)
- Node.Capacity (optional, not a limit); available inbounds report load
  percent of capacity or active config count, sorted least loaded first

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/src/PnvPanel.Infrastructure/Persistence/Configurations/InboundConfiguration.cs

@@ -15,6 +15,7 @@ public class InboundConfiguration : IEntityTypeConfiguration<Inbound>
         builder.Property(x => x.Protocol).HasConversion<string>().HasMaxLength(32);
         builder.Property(x => x.Remark).IsRequired().HasMaxLength(200);
         builder.Property(x => x.DisplayName).HasMaxLength(100);
+        builder.Property(x => x.CountryCode).HasMaxLength(2);
 
         builder.Property(x => x.AllowedRoleIds).HasColumnType("uuid[]");