feat(backup): add chain-metadata keys + nas.backup.full.every config

NASBackupChainKeys defines the keys this provider stores under the
existing backup_details kv table (parent_backup_id, bitmap_name,
chain_id, chain_position, type). This keeps the backups table
provider-agnostic per the RFC review.

nas.backup.full.every is a zone-scoped ConfigKey that controls how
often a full backup is taken; the remaining backups in the cycle are
incremental. Counts backups (not days), so it works for hourly,
daily, and ad-hoc schedules. Default 10. Set to 1 to disable
incrementals (every backup is full).

Refs: apache/cloudstack#12899

plugins/backup/nas/src/main/java/org/apache/cloudstack/backup/NASBackupChainKeys.java

@@ -0,0 +1,47 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+package org.apache.cloudstack.backup;
+
+/**
+ * Keys used by the NAS backup provider when storing incremental-chain metadata
+ * in the existing {@code backup_details} key/value table. Stored here (not on
+ * the {@code backups} table) so other providers do not need a schema change to
+ * support their own incremental implementations.
+ */
+public final class NASBackupChainKeys {
+
+    /** UUID of the parent backup (full or previous incremental). Empty for full backups. */
+    public static final String PARENT_BACKUP_ID = "nas.parent_backup_id";
+
+    /** QEMU dirty-bitmap name created by this backup, used as the {@code <incremental>} reference for the next one. */
+    public static final String BITMAP_NAME = "nas.bitmap_name";
+
+    /** Identifier shared by every backup in the same chain (the full anchors a chain; its incrementals inherit the id). */
+    public static final String CHAIN_ID = "nas.chain_id";
+
+    /** Position within the chain: 0 for the full, 1 for the first incremental, and so on. */
+    public static final String CHAIN_POSITION = "nas.chain_position";
+
+    /** Backup type marker: {@value #TYPE_FULL} or {@value #TYPE_INCREMENTAL}. Mirrors {@code backups.type} for fast lookup without a join. */
+    public static final String TYPE = "nas.type";
+
+    public static final String TYPE_FULL = "full";
+    public static final String TYPE_INCREMENTAL = "incremental";
+
+    private NASBackupChainKeys() {
+    }
+}

plugins/backup/nas/src/main/java/org/apache/cloudstack/backup/NASBackupProvider.java

@@ -85,6 +85,16 @@ public class NASBackupProvider extends AdapterBase implements BackupProvider, Co
             true,
             BackupFrameworkEnabled.key());
 
+    ConfigKey<Integer> NASBackupFullEvery = new ConfigKey<>("Advanced", Integer.class,
+            "nas.backup.full.every",
+            "10",
+            "Take a full NAS backup every Nth backup; remaining backups in between are incremental. " +
+                    "Counts backups, not days, so it works for hourly, daily, and ad-hoc schedules. " +
+                    "Set to 1 to disable incrementals (every backup is full).",
+            true,
+            ConfigKey.Scope.Zone,
+            BackupFrameworkEnabled.key());
+
     @Inject
     private BackupDao backupDao;
 
@@ -629,7 +639,8 @@ public class NASBackupProvider extends AdapterBase implements BackupProvider, Co
     @Override
     public ConfigKey<?>[] getConfigKeys() {
         return new ConfigKey[]{
-                NASBackupRestoreMountTimeout
+                NASBackupRestoreMountTimeout,
+                NASBackupFullEvery
         };
     }