feat(ovpnrw): connections history from storage if available

[m-dilorenzi]

This pull request introduces several improvements and enhancements to the OpenVPN connections history section, primarily focused on supporting persistent storage for connection history databases, improving API functionality, and adding mechanisms to synchronize connection records when storage is mounted.
The changes ensure that connection history is preserved across reboots, provide more flexible API filtering and sorting, and add scripts to automate database migration.

Persistent storage and database path handling:

• All relevant code now checks for the presence of /mnt/data/openvpn and uses it as the preferred location for connections.db, falling back to /var/openvpn if storage is not available. Helper functions like get_connections_db_path and get_ovpnrw_db_path were introduced to centralize this logic.
• The documentation (README.md) was updated to clarify that connection history is stored in either /var/openvpn/<instance>/connections.db or /mnt/data/openvpn/<instance>/connections.db depending on storage configuration.

Database migration and archiving:

• A new hotplug script (20-openvpn-merge-connections-db) and migration utility (openvpn-merge-connections-db) were added to synchronize connection records from /var to /mnt/data when storage is mounted, ensuring persistence across reboots.
• When removing an OpenVPN instance, the connection history database is now archived before deletion to prevent data loss.

API enhancements and reporting improvements:

• The connection_history API was refactored to support filtering (by time range, accounts, and search query), sorting (including manual sorting for IP addresses), and pagination, returning richer metadata for frontend use.
• All reporting functions now use the new database path resolution logic, ensuring data is pulled from the correct location regardless of storage configuration.

These changes collectively improve reliability, flexibility, and maintainability of the OpenVPN connections history section.

Closes: #1404