feat(deploy): ausgehender HTTP(S)-Proxy für Build und Laufzeit

- Compose reicht HTTP_PROXY/HTTPS_PROXY/NO_PROXY als Build-Argumente an beide Images weiter
  (npm ci, Google Fonts) und zur Laufzeit an api und worker; api, db, web stehen immer in NO_PROXY
- Spielerköpfe werden über undici EnvHttpProxyAgent geladen (Node-fetch ignoriert Proxy-Variablen)
- Doku: Proxy-Abschnitt mit Whitelist und Fehlerbildern
- geprüft mit lokalem Squid: Build nur über registry.npmjs.org, fonts.googleapis.com, fonts.gstatic.com;
  zur Laufzeit nur mc-heads.net

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

deploy/docker-compose.yml

@@ -7,15 +7,31 @@
 
 name: atm10-stats
 
+# Outbound proxy (e.g. Squid in the DMZ). The Docker daemon's own proxy setting only covers image pulls;
+# RUN steps in the build (npm, Google Fonts) and the running API need it passed explicitly.
+x-proxy-build-args: &proxy-build-args
+  HTTP_PROXY: ${HTTP_PROXY:-}
+  HTTPS_PROXY: ${HTTPS_PROXY:-}
+  NO_PROXY: ${NO_PROXY:-}
+  http_proxy: ${HTTP_PROXY:-}
+  https_proxy: ${HTTPS_PROXY:-}
+  no_proxy: ${NO_PROXY:-}
+
 x-backend: &backend
   build:
     context: ..
     dockerfile: backend/Dockerfile
+    args: *proxy-build-args
   image: atm10-stats/backend:latest
   restart: unless-stopped
   env_file: ${ENV_FILE:-.env}
   environment:
     DATABASE_URL: postgres://atm:${POSTGRES_PASSWORD}@db:5432/atm
+    # Player heads (mc-heads.net) go through the proxy; Compose-internal names and the database never do.
+    # SFTP to the game server is a direct SSH connection and does not use these variables.
+    HTTP_PROXY: ${HTTP_PROXY:-}
+    HTTPS_PROXY: ${HTTPS_PROXY:-}
+    NO_PROXY: ${NO_PROXY:-localhost,127.0.0.1},api,db,web
   depends_on:
     db:
       condition: service_healthy
@@ -71,6 +87,7 @@ services:
     build:
       context: ..
       dockerfile: web/Dockerfile
+      args: *proxy-build-args
     image: atm10-stats/web:latest
     restart: unless-stopped
     environment: