Add Steam API client, service, controller

Introduce Steam integration: add SteamClientConfig (RestClient bean), SteamApiService (calls GetPlayerSummaries and returns a Player DTO), SteamPlayerSummaryResponse DTO, and SteamTestController (/api/steam/test/{steamId}) for quick testing. Update SecurityConfig to permit the test endpoint and adjust the SecurityFilterChain bean signature. Add Steam API settings: base-url in application.yml and a dev API key in application-dev.yml. Includes basic error logging and optional response handling when no player is found.

src/main/resources/application.yml

@@ -42,6 +42,7 @@ app:
     allowed-origins: "https://tbp.andreas-dahm.eu"
   steam:
     api-key: ${STEAM_API_KEY}
+    base-url: https://api.steampowered.com
     playerService-url: https://api.steampowered.com/IPlayerService/GetOwnedGames/v1/
     storeService-url: https://api.steampowered.com/IStoreService/GetAppList/v1/
     imageService-url: https://cdn.cloudflare.steamstatic.com/steam/apps/