fix(hosts): assign group ids to imported hosts and repair empty ones

Host rows created from a legacy streamSettings.externalProxy during
inbound import got an empty group_id, and the one-time HostGroupIds
seeder had already been gated off, so the UI rendered them under a
synthetic fallback_<id> group the update/delete API could not resolve,
failing every edit with "host group not found".

Assign a real group id in externalProxyEntryToHost at creation, and
replace the seeder with backfillEmptyHostGroupIds, an idempotent
startup repair that runs on every boot so rows from older builds and
restored backups are healed too. Also rename the leaked internal
error "host group not found" to "host not found" since groups are not
a user-facing concept.

internal/database/db.go

@@ -686,6 +686,7 @@ func externalProxyEntryToHost(inboundId, index int, ep map[string]any) *model.Ho
 	fingerprint, _ := ep["fingerprint"].(string)
 	ech, _ := ep["echConfigList"].(string)
 	return &model.Host{
+		GroupId:              random.NumLower(16),
 		InboundId:            inboundId,
 		SortOrder:            index,
 		Remark:               remark,
@@ -1180,7 +1181,7 @@ func runSeeders(isUsersEmpty bool) error {
 		return err
 	}
 
-	if err := seedHostGroupIds(); err != nil {
+	if err := backfillEmptyHostGroupIds(); err != nil {
 		return err
 	}
 
@@ -1213,36 +1214,27 @@ func seedNodeInboundsAdopted() error {
 	return db.Create(&model.HistoryOfSeeders{SeederName: "NodeInboundsAdopted"}).Error
 }
 
-func seedHostGroupIds() error {
-	var history []string
-	if err := db.Model(&model.HistoryOfSeeders{}).Pluck("seeder_name", &history).Error; err != nil {
-		return err
-	}
-	if slices.Contains(history, "HostGroupIds") {
-		return nil
-	}
-
+// backfillEmptyHostGroupIds is idempotent and not seeder-gated: builds that
+// predate group ids on the inbound-import path (and restored backups) can
+// re-introduce hosts rows with an empty group_id, and such rows render as a
+// synthetic fallback_<id> group the update/delete API cannot address, so
+// re-check on every start.
+func backfillEmptyHostGroupIds() error {
 	var hosts []*model.Host
 	if err := db.Where("group_id = '' OR group_id IS NULL").Find(&hosts).Error; err != nil {
 		return err
 	}
-
-	if len(hosts) > 0 {
-		err := db.Transaction(func(tx *gorm.DB) error {
-			for _, h := range hosts {
-				h.GroupId = random.NumLower(16)
-				if err := tx.Model(h).Update("group_id", h.GroupId).Error; err != nil {
-					return err
-				}
-			}
-			return nil
-		})
-		if err != nil {
-			return err
-		}
+	if len(hosts) == 0 {
+		return nil
 	}
-
-	return db.Create(&model.HistoryOfSeeders{SeederName: "HostGroupIds"}).Error
+	return db.Transaction(func(tx *gorm.DB) error {
+		for _, h := range hosts {
+			if err := tx.Model(h).Update("group_id", random.NumLower(16)).Error; err != nil {
+				return err
+			}
+		}
+		return nil
+	})
 }
 
 func resetIpLimitsWithoutFail2ban() error {