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/subJsonService.go

@@ -221,7 +221,7 @@ func (s *SubJsonService) getConfig(inbound *model.Inbound, client model.Client,
 			newOutbounds = append(newOutbounds, s.genVless(inbound, streamSettings, client))
 		case "trojan", "shadowsocks":
 			newOutbounds = append(newOutbounds, s.genServer(inbound, streamSettings, client))
-		case "hysteria", "hysteria2":
+		case "hysteria":
 			newOutbounds = append(newOutbounds, s.genHy(inbound, newStream, client))
 		}