Soft-deleted conversations remain accessible through console APIs

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### Dify version

main

### Cloud or Self Hosted

Self Hosted (Source)

### Steps to reproduce

1. Run Dify from the current `main` branch.
2. Create a chat conversation and add at least one message.
3. Delay or stop the asynchronous conversation cleanup worker.
4. Delete the conversation from the console.
5. Request the deleted conversation through `GET /console/api/apps/{app_id}/chat-conversations/{conversation_id}`.
6. Request its messages through `GET /console/api/apps/{app_id}/chat-messages?conversation_id={conversation_id}`.

The delete path commits `Conversation.is_deleted = true` before scheduling physical cleanup. The conversation list already excludes these rows, but both direct read queries omit the soft-delete predicate.

### ✔️ Expected Behavior

Once deletion succeeds, the conversation detail and message history endpoints should return 404 immediately, regardless of asynchronous cleanup progress.

### ❌ Actual Behavior

Until asynchronous cleanup physically removes the row, both endpoints can still return the soft-deleted conversation and its messages. This can be especially visible when the cleanup queue is delayed or unavailable.

Reproduce the issue with the asynchronous cleanup worker delayed, then inspect the direct-read queries behind the conversation detail and message-history endpoints. Compare them with the conversation list query, which already excludes soft-deleted rows. Done means both endpoints return 404 immediately after deletion, before physical cleanup occurs.

Written by the indexing model from the issue text.