Ensure tgadmin runtime log and data dirs are created in postinst

[Al-Muhandis]

Motivation

• Prevent the daemon from failing at startup when /var/log/tgadmin or /var/lib/tgadmin do not exist by creating those runtime directories during package installation and assigning correct owner/group and permissions.

Description

• Add LOG_DIR and DATA_DIR variables to debian/DEBIAN/postinst.
• Add ensure_runtime_directories() which creates the log and data directories with install -d -m 0750 -o "${SERVICE_USER}" -g "${SERVICE_GROUP}" ... so the service user owns them and they are not world-readable.
• Call ensure_runtime_directories during the configure path after ensure_service_account and before systemd drop-in configuration.

Testing

• Ran bash -n debian/DEBIAN/postinst to validate the maintainer script syntax, which succeeded.

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Codex Task