Add timezone support for backup scheduling

[mrwogu]

Overview

This PR adds robust timezone support for scheduled backups.

Features

• Added support for the TZ environment variable to control the timezone used for scheduled backups.
• If TZ is set, all scheduled backups are interpreted in the specified timezone. If not set, the system default timezone is used.
• If an invalid timezone is provided, the process will exit with an error message.
• The backup startup message now displays the active timezone for clarity.

Implementation

• Introduced the use of pytz to handle timezone conversion in the backup process.
• Updated the backup entrypoint and scheduling logic to print the current value of TZ.
• Added a dedicated test (test_backup_timezone) to verify that the backup logic correctly applies the timezone from the TZ environment variable.

Documentation

• Added a new "Timezone support" section to the README.md, including usage examples for setting the TZ environment variable in your environment or docker-compose.yml.

Testing

• Added a comprehensive test for timezone support in backup scheduling.

Use Case

This addresses the use case where users need to run scheduled backups in specific timezones, ensuring correct backup timing regardless of the system locale.

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Closes #77