feat(sub): generate random IPs from CIDR host addresses (#6784)

Operators who front a server with an IP range (a CDN or anycast prefix) had to
list each address as its own host. A host address may now be a CIDR, and every
subscription fetch expands it into up to randomCount (1–100, default 1) unique
random host IPs from the range, IPv4 and IPv6 alike, skipping the network and
broadcast addresses. The panel's own previews, QR codes and tunnel config
exports (WireGuard, AmneziaWG, TUIC, MTProto) expand it the same way, and the
externalProxy importer writes the model's default count so the re-sync still
recognises untouched imported hosts.

internal/web/entity/entity.go

@@ -447,4 +447,5 @@ type HostGroup struct {
 	MihomoIpVersion        string   `json:"mihomoIpVersion" validate:"omitempty,oneof=dual ipv4 ipv6 ipv4-prefer ipv6-prefer"`
 	MihomoX25519           bool     `json:"mihomoX25519"`
 	ShuffleHost            bool     `json:"shuffleHost"`
+	RandomCount            int      `json:"randomCount" validate:"omitempty,min=1,max=100"`
 }