feat(sub): per-inbound sort order for subscription links

Add a subSortIndex field to inbounds that controls the order of links
in subscription output only: the raw sub body, the HTML sub page, and
the JSON/Clash formats (all served from the same query). Lower values
come first; ties keep id order. The panel inbound list is unaffected.

The value is editable in the inbound form next to the share-address
fields, propagates to nodes via wireInbound, and follows the usual
node-sync rules (copied on import, mirrored while not dirty, never a
structural change).

Rescoped from #5214 by @Ponywka.

internal/web/service/inbound_util.go

@@ -7,6 +7,16 @@ package service
 // installs (>32k clients) where even modern SQLite would refuse a single IN.
 const sqliteMaxVars = 900
 
+// normalizeSubSortIndex clamps the 1-based subscription sort order. Values
+// below 1 arrive from clients that predate the field (omitted form key binds
+// to 0) and must not sort ahead of explicitly ranked inbounds.
+func normalizeSubSortIndex(v int) int {
+	if v < 1 {
+		return 1
+	}
+	return v
+}
+
 // uniqueNonEmptyStrings returns a deduplicated copy of in with empty strings
 // removed, preserving the order of first occurrence.
 func uniqueNonEmptyStrings(in []string) []string {