chore: STAGING_PATH entfernt, ci-starter-defaults für Template-Staging

- STAGING_PATH aus .env und Makefile (media-sync-local-to-staging) entfernt, Doku bereinigt
- scripts/ci-starter-defaults.sh löst __PLACEHOLDER__ im CI-Workspace auf, dev/master bleiben identisch

scripts/ci-staging.sh

@@ -5,6 +5,11 @@ set -euo pipefail
 # Called from .gitea/workflows/deploy.yml
 # Required env: API_BASEDIR, COMPOSE_PROJECT_NAME
 
+# Starter template: resolve __PLACEHOLDER__ to CI workspace values (no-op for real projects)
+if [ -f ./scripts/ci-starter-defaults.sh ]; then
+    ./scripts/ci-starter-defaults.sh
+fi
+
 # Read .env
 set -o allexport
 . ./.env