Previously, proxy hosts with "Managed by Caddy (Auto)" (certificate_id = null)
were being skipped during Caddy configuration generation, causing the feature
to not work at all.
This commit adds full support for automatic certificate management:
1. Modified collectCertificateUsage() to track domains with null certificate_id
separately as auto-managed domains
2. Updated buildTlsAutomation() to create ACME automation policies for
auto-managed domains (supports both HTTP-01 and DNS-01 challenges)
3. Modified buildTlsConnectionPolicies() to include TLS connection policies
for auto-managed domains
4. Updated buildProxyRoutes() to allow proxy hosts with null certificate_id
to be included in the route configuration
The configuration now automatically updates when domains are changed, as
applyCaddyConfig() is already called on create/update/delete operations.
Caddy will now automatically obtain and manage Let's Encrypt certificates
for all domains when "Managed by Caddy (Auto)" is selected.
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