fix(ssl): block ssl-verify on non-letsencrypt sites

[mrrobot47]

Problem

ee site ssl-verify (ssl_verify()) had no guard on the site's SSL type. Run on a custom / self / inherit site it would still prompt for a Let's Encrypt email, instantiate the ACME client, run the challenge, and request an LE certificate — which overwrites the user's custom certificate if the domain validates, or yields a confusing "Failed to verify SSL" error with a retry hint that can never succeed for a non-LE cert.

Fix

Guard ssl_verify() to error clearly when site_ssl !== 'le'. The guard sits after site_data is populated (so it covers the user-invoked subcommand) and does not affect the internal call from init_le() during LE site creation — that path is only reached when site_ssl === 'le', so the guard never fires there.

Testing

Manual:

1. ee site create custom.test --type=html --ssl=custom --ssl-key=... --ssl-crt=...
2. ee site ssl-verify custom.test → exits with "SSL verification is only applicable to Let's Encrypt certificates." (no email prompt, no ACME run, certs untouched).
3. Regression: an --ssl=le site still verifies/renews as before.

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