fix(security): keep database passwords out of every argv

MariaDB dumps and restores read the password from MYSQL_PWD, and docker exec forwards env variables by name only, so no process listing shows a password. baudolo-restore gains --db-password-stdin; --db-password still works but warns.

🎯 Fix: 96 %
💥 Break: 3 %
💸 Cost: –
⏱️ Time: –
🛡️ Security: ~100 %
🏆 Score: 93
🚦 Gate: ✅ 90/5, cost and time <= 100, security 100 %

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

src/baudolo/backup/db.py

@@ -132,11 +132,12 @@ def backup_database(
                         "--protocol=tcp",
                         "-u",
                         user,
-                        f"-p{password}",
                         db_name,
                     ],
+                    forward_env=["MYSQL_PWD"],
                 ),
                 dump_file,
+                env={"MYSQL_PWD": password},
             )
             produced = True
             continue