feat(simnet): add SimNet protocol end-to-end support

- model: SimNet protocol fields + NormalizeSimnet; type+port protocol uniqueness
- server config: OmnXT runtime config delivery via compatible() simnet case
- credentials: derive per-user psk/key_id from subscription (pkg/simnet), no new table
- subscription: adapter buildOmnxtSimnetConfigs + base64 buildOmnxtProtocolLinks + OmnXT SimNet application (migration 02161)
- UA gating: hide experimental protocols from non first-party clients (download + JSON node-list)
- admin: normalize simnet on create/update and on GET responses
- tests: 21 simnet unit tests; full suite green

internal/logic/server/constant.go

@@ -12,6 +12,7 @@ const (
 	// Deprecated: Hysteria2 is deprecated, use Hysteria instead
 	// TODO: remove in future versions
 	Hysteria2 = "hysteria2"
+	Simnet    = "simnet"
 )
 
 type SecurityConfig struct {