Refactor logging and drop deprecated FLASK_ENV

Move logging setup into max/log.py. The file handler is only used outside
debug mode and its directory is configurable via LOG_DIR. FLASK_ENV is
deprecated in Flask; debug mode is now controlled via FLASK_DEBUG.

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

.devcontainer/compose.yml

@@ -9,8 +9,6 @@ services:
       - home:/home/dev
     environment:
       - PYTHONPATH=/workspaces/max
-      - FLASK_APP=max.app
-      - FLASK_ENV=development
       - PGSQL_HOST=postgresql
       - PGSQL_PORT=5432
       - PGSQL_USER=max