Support non-empty directory as datadir

[ManuelBilbao]

EthDocker puts the JWT among other stuff in the same directory as the datadir, causing an error as the code expect an empty dir (here:)

ethrex/crates/storage/store.rs

Lines 2849 to 2850 in 2f60e7e

	// If datadir exists but is not empty, this is probably a DB for an
	// old ethrex version and we should return an error

We should do a better check for old DB existance and allow non-empty dirs

[MegaRedHand]

Alt: we could store the DB inside <datadir_path>/data. Would be cleaner and more difficult for users to trip on

[pjdurden]

I'd like to take this. The check at store.rs#L2849-L2850 treats any non-empty datadir as an error, but the real intent is "don't clobber an existing DB." Plan: instead of asserting the dir is empty, probe for the actual DB marker (libmdbx mdbx.dat / the rocksdb dir, per configured backend) and only refuse when a real prior DB exists — otherwise proceed so unrelated files (JWT, etc.) can coexist, which is what EthDocker needs. I'll add tests for (a) empty dir, (b) dir with only a JWT, (c) dir with an existing DB. Happy to align on the exact marker per backend before I open the PR.