Previously, ACME keys were stored at a single default path regardless of
which CA directory URL was configured. This caused key conflicts when
using multiple different ACME CAs.
Now, the key path is derived from a SHA256 hash of the CA directory URL,
allowing each CA to have its own key file:
- Default CA (Let's Encrypt): certs/acme.key
- Custom CA: certs/acme_<url_hash_16chars>.key
This enables running certificates against multiple ACME providers without
key collision issues.
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