fix: create some compat aliases for services that look for config

[samrose]

will follow up with PR on these services to change where they look

The PostgreSQL configuration was refactored to use include_dir = '/etc/postgresql-custom/conf.d' which auto-includes all .conf files from that directory. This replaced explicit include statements for individual config files.

However, other services (admin-mgr, supabase-admin-api) still write configuration to the old paths in the parent directory (/etc/postgresql-custom/). Files written there are not auto-included by PostgreSQL.

Added symlinks to maintain backward compatibility:

• /etc/postgresql-custom/read-replica.conf → conf.d/read_replica.conf
  • admin-mgr writes to the old path when configuring read replicas
• /etc/postgresql-custom/custom-overrides.conf → conf.d/custom_overrides.conf
  • supabase-admin-api writes custom PostgreSQL settings to the old path

These symlinks allow other services to continue using their existing paths while ensuring the configuration is actually loaded via the new include_dir mechanism.