Remove unused hostname_prefix from server types and add duplicate checking

The HostnamePrefix field on ServerType was loaded from YAML but never used —
hostnames are user-provided. This removes the field and adds explicit
duplicate checks (hostname + MAC) with clear per-field error messages in
both the JSON API and web UI, backed by a new GetByHostname store method
with case-insensitive matching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

deploy/server-types.example.yaml

@@ -4,7 +4,6 @@ server_types:
     boot_disk: "/dev/nvme0n1"
     management_nic: "enp2s0"
     gpu: false
-    hostname_prefix: "pve-ms"
     expected_nics:
       - name: "enp2s0"
         speed: "2500"
@@ -16,7 +15,6 @@ server_types:
     boot_disk: "/dev/nvme0n1"
     management_nic: "enp1s0"
     gpu: true
-    hostname_prefix: "pve-um"
     expected_nics:
       - name: "enp1s0"
         speed: "2500"