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.

Sites with SSL not enabled get a specific error instead: SSL is not enabled on <site>. Enable Let's Encrypt SSL with `ee site update <site> --ssl=le`. (with --wildcard for wildcard sites). That is the state a failed Let's Encrypt setup at create time leaves behind (an empty site_ssl, plus a hint to re-run ssl-verify), and ssl-verify can't complete that recovery because it never sets the SSL flag; ee site update --ssl=le does.

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.
4. On a site without SSL, ee site ssl-verify <site> → SSL is not enabled on <site>. Enable Let's Encrypt SSL with `ee site update <site> --ssl=le`.

Tested on Ubuntu 26.04 with EasyEngine 4.12.0: custom, self-signed, inherit and no-SSL sites (exit 1, no email prompt, no ACME traffic, cert files unchanged).

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