[DPE-9049] feat: implement automatic cleanup of lost+found storage directories

[marceloneppel]

Issue

When deploying Charmed PostgreSQL with a dedicated storage volume (e.g., NVMe drives formatted with ext4), the filesystem automatically creates a lost+found directory at the root of the mount point. initdb treats the storage directory as non-empty and refuses to initialise the database cluster, resulting in an error such as:

initdb: error: directory "/var/snap/charmed-postgresql/common/var/lib/postgresql" exists but is not empty
initdb: detail: It contains a lost+found directory, perhaps due to it being a mount point.
initdb: hint: Using a mount point directly as the data directory is not recommended.

This prevents the PostgreSQL cluster from ever forming without manual intervention.

Solution

The charm now automatically removes any lost+found directory found at the root of each storage path (data, archive, logs, and temp) during the install and start events, eliminating the need for external workarounds.

Tested with ext4 volumes in a Testflinger machine (it's needed to use LXD VMs instead of containers for it to work).

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

Fixes #1336.

[taurus-forever]

For the history, we cannot rename/move even the PostgreSQL does ccare about dot_files and lost+found here: https://github.com/postgres/postgres/blob/66ad764c8d517f59577d41ac3dad786729c9e10e/src/port/pgcheckdir.c#L60

it still aborts basebackup execution in https://github.com/postgres/postgres/blob/66ad764c8d517f59577d41ac3dad786729c9e10e/src/bin/pg_basebackup/pg_basebackup.c#L771-L772

This is a new Juju volume which obviously has empty lost+found folder, so we are cleaning it to have deployment fixed. The folder will be re-created automatically.

[delgod]

I think we can be smarter here, clean up the directory only if initdb is run nessesary.
The problem right now is that if we have a healthy PostgreSQL and the kernel crashes, and the machine is restarted, it can get a valid lost+found folder needed for recovery.