3.2.11: ntfy authentication support

Adds username/password authentication for access-controlled ntfy topics, including self-hosted servers and reserved topics.

• Configure optional ntfy credentials in notification settings.
• Credentials are saved and used for both test notifications and scheduled subscription reminders.
• Existing databases are migrated automatically.
• Includes the notification connectivity and diagnostics improvements from the 3.2.10 prerelease.

Thanks to @nicolast654 for contributing #38.

Validation: production build passed; all 21 automated tests passed with Node 20.

Note: ntfy credentials are stored in plaintext in the application database, consistent with existing Gotify token storage.

The release workflow builds and tests Linux amd64 and arm64 containers before publishing dh1011/subscription-manager:3.2.11 and updating latest.