feat: add ephemeral retention mode for forced temporary chats

Extends the `retentionMode` interface option with a new `ephemeral` value
that forces every conversation to be temporary and applies expiration
deadlines to all data. Unlike `all` (which keeps chats visible until they
expire), `ephemeral` marks all chats temporary so they are hidden from
history and search, with the per-chat toggle locked on.

- Add RetentionMode.EPHEMERAL plus isAllDataRetention and
  isForcedTemporaryRetention helpers
- Force isTemporary and expiry on conversations, messages, imports, and files
- Force the TEMPORARY_CHAT permission on so the locked indicator is shown
- Lock the temporary-chat toggle in the UI and force new/existing chats temporary
- Document the new mode in librechat.example.yaml

api/server/utils/import/importBatchBuilder.js

@@ -7,8 +7,9 @@ const {
 const {
   EModelEndpoint,
   Constants,
-  RetentionMode,
   openAISettings,
+  isAllDataRetention,
+  isForcedTemporaryRetention,
 } = require('librechat-data-provider');
 const { bulkIncrementTagCounts, bulkSaveConvos, bulkSaveMessages } = require('~/models');
 const { FALLBACK_MODEL_BY_ENDPOINT } = require('./defaults');
@@ -45,19 +46,20 @@ class ImportBatchBuilder {
       return this.retentionFields;
     }
 
-    if (this.interfaceConfig?.retentionMode !== RetentionMode.ALL) {
+    if (!isAllDataRetention(this.interfaceConfig?.retentionMode)) {
       this.retentionFields = {};
       return this.retentionFields;
     }
 
+    const isTemporary = isForcedTemporaryRetention(this.interfaceConfig?.retentionMode);
     try {
       this.retentionFields = {
-        isTemporary: false,
+        isTemporary,
         expiredAt: createTempChatExpirationDate(this.interfaceConfig),
       };
     } catch (error) {
       logger.error('[ImportBatchBuilder] Error creating import expiration date:', error);
-      this.retentionFields = { isTemporary: false, expiredAt: createFallbackRetentionDate() };
+      this.retentionFields = { isTemporary, expiredAt: createFallbackRetentionDate() };
     }
     return this.retentionFields;
   }

api/server/utils/import/importers.spec.js

@@ -1145,6 +1145,23 @@ describe('importLibreChatConvo', () => {
       expect(result.conversation.expiredAt).toBeInstanceOf(Date);
       expect(result.conversation.expiredAt).toBe(message.expiredAt);
     });
+
+    it('marks imported conversations and messages temporary under ephemeral retention', () => {
+      const requestUserId = 'user-123';
+      const builder = new ImportBatchBuilder(requestUserId, {
+        retentionMode: RetentionMode.EPHEMERAL,
+        temporaryChatRetention: 24,
+      });
+      builder.startConversation(EModelEndpoint.openAI);
+      const message = builder.addUserMessage('Ephemeral import');
+      const result = builder.finishConversation('Imported ephemeral chat');
+
+      expect(message.isTemporary).toBe(true);
+      expect(message.expiredAt).toBeInstanceOf(Date);
+      expect(result.conversation.isTemporary).toBe(true);
+      expect(result.conversation.expiredAt).toBeInstanceOf(Date);
+      expect(result.conversation.expiredAt).toBe(message.expiredAt);
+    });
   });
 });