fix(staging): hardcode APP_DEBUG=false to prevent env override

Changed APP_DEBUG from ${APP_DEBUG:-false} to hardcoded false value
in all 4 services (php, nginx, queue-worker, scheduler).

This prevents any server-side .env or environment variables from
accidentally enabling debug mode in staging, which was causing
detailed error pages to be displayed.

docker-compose.staging.yml

@@ -20,7 +20,7 @@ services:
     environment:
       - TZ=Europe/Berlin
       - APP_ENV=staging
-      - APP_DEBUG=${APP_DEBUG:-false}
+      - APP_DEBUG=false
       - APP_URL=https://staging.michaelschiemer.de
       - APP_KEY=${APP_KEY:-}
       # Git Repository - clones staging branch
@@ -210,7 +210,7 @@ services:
     environment:
       - TZ=Europe/Berlin
       - APP_ENV=staging
-      - APP_DEBUG=${APP_DEBUG:-false}
+      - APP_DEBUG=false
       # Git Repository - clones staging branch
       - GIT_REPOSITORY_URL=${GIT_REPOSITORY_URL:-}
       - GIT_BRANCH=staging
@@ -351,7 +351,7 @@ services:
     environment:
       - TZ=Europe/Berlin
       - APP_ENV=staging
-      - APP_DEBUG=${APP_DEBUG:-false}
+      - APP_DEBUG=false
       # Database - using separate staging database
       - DB_HOST=postgres
       - DB_PORT=5432
@@ -406,7 +406,7 @@ services:
     environment:
       - TZ=Europe/Berlin
       - APP_ENV=staging
-      - APP_DEBUG=${APP_DEBUG:-false}
+      - APP_DEBUG=false
       # Database - using separate staging database
       - DB_HOST=postgres
       - DB_PORT=5432