feat: resolve player display names from Steam persona (Steam Web API)

Accounts were created as 'Unknown' and the client never sends a name, so the admin
panel / friends / squad showed no names. Add SteamWebApiService (GetPlayerSummaries),
wired into LoginWithSteam to backfill the DisplayName from the Steam persona when it's
still 'Unknown'. Admin gets a LAN-only /admin/backfill-names action (+ button) to
backfill existing accounts in one shot. Key read from config SteamWebApiKey (Vault ->
env), inert when unset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Prospect.Server.Api/Startup.cs

@@ -30,6 +30,8 @@ public class Startup
         services.AddSingleton<UserDataService>();
         services.AddSingleton<TitleDataService>();
 
+        services.AddSingleton<Prospect.Server.Api.Services.Steam.SteamWebApiService>();
+
         services.AddSingleton<DbUserService>();
         services.AddSingleton<DbEntityService>();