fix(hwid): track devices only for subscriptions a client owns

The subscription routes run the HWID gate before they look up the
subId, and the gate read "no device limit" and "no such subscription"
alike, so a request naming any path segment with an X-HWID header was
answered 404 yet stored that segment, up to about 1 MiB, in
client_hwids for good. Anyone reaching the subscription port could fill
the disk until the panel could no longer save anything.

An unlimited subscription is tracked only while an enabled client owns
its subId, and a one-time seeder drops the rows already stored for
subIds no client owns.

internal/database/db.go

@@ -1530,7 +1530,7 @@ func runSeeders(isUsersEmpty bool) error {
 	}
 
 	if empty && isUsersEmpty {
-		seeders := []string{"UserPasswordHash", "ClientsTable", "InboundClientsArrayFix", "InboundClientTgIdFix2", "InboundClientSubIdFix", "FreedomFinalRulesReverseFix", "FreedomFinalRulesPrivateEgressBlock", "UppercaseFreedomFinalRulesFix", "InboundRealityFinalmaskTcpStrip", "ApiTokensHash", "LegacyProxySettingsCleanup", "SessionSecretRotation", "OutboundRemovedKeysFix", "FreedomDomainStrategyFix", "DNSOutboundLegacyKeysFix", "DNSOutboundQTypeZeroFix", "WireguardDomainStrategyFix", "XdnsFinalmaskNamesAddrsFix", "UDPHopClientMaskFix", "HostVerifyPeerCertByNameBoolFix", "HostMihomoX25519Reset", "WireguardPeersToClients", "MtprotoSecretsToClients", "NodeInboundsAdopted", "NodeDownNotified", "ResetIpLimitNoFail2ban", "ClientGroupNameTrim"}
+		seeders := []string{"UserPasswordHash", "ClientsTable", "InboundClientsArrayFix", "InboundClientTgIdFix2", "InboundClientSubIdFix", "FreedomFinalRulesReverseFix", "FreedomFinalRulesPrivateEgressBlock", "UppercaseFreedomFinalRulesFix", "InboundRealityFinalmaskTcpStrip", "ApiTokensHash", "LegacyProxySettingsCleanup", "SessionSecretRotation", "OutboundRemovedKeysFix", "FreedomDomainStrategyFix", "DNSOutboundLegacyKeysFix", "DNSOutboundQTypeZeroFix", "WireguardDomainStrategyFix", "XdnsFinalmaskNamesAddrsFix", "UDPHopClientMaskFix", "HostVerifyPeerCertByNameBoolFix", "HostMihomoX25519Reset", "WireguardPeersToClients", "MtprotoSecretsToClients", "NodeInboundsAdopted", "NodeDownNotified", "ResetIpLimitNoFail2ban", "ClientGroupNameTrim", "OwnerlessClientHwidPurge"}
 		for _, name := range seeders {
 			if err := db.Create(&model.HistoryOfSeeders{SeederName: name}).Error; err != nil {
 				return err
@@ -1719,6 +1719,12 @@ func runSeeders(isUsersEmpty bool) error {
 		}
 	}
 
+	if !slices.Contains(seedersHistory, "OwnerlessClientHwidPurge") {
+		if err := purgeOwnerlessClientHwids(); err != nil {
+			return err
+		}
+	}
+
 	if err := seedHostsFromExternalProxy(); err != nil {
 		return err
 	}
@@ -1914,6 +1920,18 @@ func clearLegacyProxySettings() error {
 	})
 }
 
+// The HWID gate stored a row for any subId a request named, owned or not; those rows
+// belong to no subscription and only grow the database.
+func purgeOwnerlessClientHwids() error {
+	return db.Transaction(func(tx *gorm.DB) error {
+		if err := tx.Where("sub_id NOT IN (SELECT sub_id FROM clients WHERE sub_id IS NOT NULL)").
+			Delete(&model.ClientHwid{}).Error; err != nil {
+			return err
+		}
+		return tx.Create(&model.HistoryOfSeeders{SeederName: "OwnerlessClientHwidPurge"}).Error
+	})
+}
+
 // The groups page addresses a group by its trimmed name, so a padded stored name listed as
 // its own group while every action on it hit the trimmed one.
 func trimClientGroupNames() error {