Found during a targeted code review, not from a user report.
MQTT_PORT, POLL_INTERVAL, and the other numeric module-level shortcuts in
config_loader.py ran int(get(...)) unguarded at import time. A hand-edited
config.ini with a typo (e.g. "mqtt_port = 98833x") raised an uncaught
ValueError before the bridge even started, with a raw traceback instead of
a usable diagnostic - list_printers() already guarded this exact class of
input the same way, but the module-level constants didn't. Added
_safe_int() (same try/except-with-fallback pattern) and applied it
everywhere int() was called unguarded on a config value.
Also wrapped migrate_env_to_config()'s filesystem writes (which also run
at import time during first-run .env migration) in try/except, so a
permission or disk-full error logs a clear message before re-raising
instead of surfacing as a bare traceback pointing into configparser.
New tests in tests/test_config_loader_robustness.py cover both, including
an end-to-end subprocess test that imports config_loader against a
malformed config.ini (a plain re-import wouldn't re-exercise the
import-time code path due to Python's module caching).
