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.

tools/openapigen/emit_zod.go

@@ -103,15 +103,17 @@ func applyZodValidations(expr string, t TypeRef, rules []ValidateRule) string {
 				expr += fmt.Sprintf(".lt(%s)", r.Param)
 			}
 		case "min":
-			if t.Kind == KindString {
+			switch t.Kind {
+			case KindString:
 				expr += fmt.Sprintf(".min(%s)", r.Param)
-			} else if t.Kind == KindInt || t.Kind == KindNumber {
+			case KindInt, KindNumber:
 				expr += fmt.Sprintf(".min(%s)", r.Param)
 			}
 		case "max":
-			if t.Kind == KindString {
+			switch t.Kind {
+			case KindString:
 				expr += fmt.Sprintf(".max(%s)", r.Param)
-			} else if t.Kind == KindInt || t.Kind == KindNumber {
+			case KindInt, KindNumber:
 				expr += fmt.Sprintf(".max(%s)", r.Param)
 			}
 		case "url":

tools/openapigen/schema.go

@@ -64,7 +64,7 @@ func parseStructTag(raw string) (json string, validate string, gormHasDash bool)
 	json = tag.Get("json")
 	validate = tag.Get("validate")
 	if g := tag.Get("gorm"); g != "" {
-		for _, part := range strings.Split(g, ";") {
+		for part := range strings.SplitSeq(g, ";") {
 			if strings.TrimSpace(part) == "-" {
 				gormHasDash = true
 			}
@@ -95,17 +95,17 @@ func parseValidateTag(tag string) []ValidateRule {
 		return nil
 	}
 	var rules []ValidateRule
-	for _, part := range strings.Split(tag, ",") {
+	for part := range strings.SplitSeq(tag, ",") {
 		part = strings.TrimSpace(part)
 		if part == "" {
 			continue
 		}
-		eq := strings.IndexByte(part, '=')
-		if eq < 0 {
+		before, after, ok := strings.Cut(part, "=")
+		if !ok {
 			rules = append(rules, ValidateRule{Name: part})
 			continue
 		}
-		rules = append(rules, ValidateRule{Name: part[:eq], Param: part[eq+1:]})
+		rules = append(rules, ValidateRule{Name: before, Param: after})
 	}
 	return rules
 }