refactor(backend): retire hysteria2 as a top-level protocol

Hysteria v2 is not a separate xray protocol — it is plain "hysteria"
with streamSettings.version = 2. The frontend already dropped hysteria2
from the protocol enum in 5a90f7e3; the backend was still carrying the
literal as a compat alias.

Removed:
- model.Hysteria2 constant
- model.IsHysteria helper (only callers were buildProxy + genHysteriaLink)
- TestIsHysteria
- "hysteria2" from the Inbound.Protocol validate oneof enum
- All `case model.Hysteria, model.Hysteria2:` and `case "hysteria",
  "hysteria2":` branches across client.go, inbound.go, outbound.go,
  xray.go, port_conflict.go, xray/api.go, subService.go,
  subJsonService.go, subClashService.go
- Stale #4081 comments

Kept (correctly — these are client-side URI/config schemes that are
independent of the xray protocol type):
- hysteria2:// share-link URI in subService.genHysteriaLink
- "hysteria2" Clash proxy type in subClashService.buildHysteriaProxy
- Comments referring to Hysteria v2 as a transport version

Note: this change does not include a DB migration. Existing rows with
protocol = 'hysteria2' will fall through to the default switch arms
after upgrade. A separate `UPDATE inbounds SET protocol = 'hysteria'
WHERE protocol = 'hysteria2'` is required for installs that still hold
legacy data.

sub/subClashService.go

@@ -170,9 +170,8 @@ func (s *SubClashService) getProxies(inbound *model.Inbound, client model.Client
 
 func (s *SubClashService) buildProxy(inbound *model.Inbound, client model.Client, stream map[string]any, extraRemark string) map[string]any {
 	// Hysteria has its own transport + TLS model, applyTransport /
-	// applySecurity don't fit. IsHysteria also covers the literal
-	// "hysteria2" protocol string (#4081).
-	if model.IsHysteria(inbound.Protocol) {
+	// applySecurity don't fit.
+	if inbound.Protocol == model.Hysteria {
 		return s.buildHysteriaProxy(inbound, client, extraRemark)
 	}