fix(github-source): refresh state after unchanged syncs

[AoHanBei]

Do you need to file an issue?

• I have searched the existing issues and this bug is not already filed.
• I believe this is a legitimate bug, not just a question or feature request.

Describe the bug

GitHub knowledge sources do not refresh their persisted sync state when the remote branch still points to the previously synced commit.

In deeptutor/services/github_source/sync.py, sync_source() returns immediately when old_sha == latest_sha:

if old_sha and old_sha == latest_sha:
    return SyncResult(ok=True, skipped=True)

This successful remote check does not update last_synced_at, set last_sync_status back to success, or clear a previous last_sync_error.
The background service uses last_synced_at to determine whether a source is stale and checks sources every hour. Once an unchanged source becomes older than the 24-hour freshness window, every subsequent hourly cycle checks GitHub again but leaves the source stale.
A previous transient sync error also remains visible even after a later GitHub check succeeds and confirms that the source is current.
The web-source sync path already persists a fresh success state after an unchanged crawl, so the two source types currently behave differently.

Steps to reproduce

1. Add a GitHub source to a knowledge base and complete an initial sync.
2. Leave the remote branch unchanged so that its current SHA still matches last_synced_sha.
3. Make the persisted last_synced_at older than 24 hours. Optionally leave last_sync_status="error" and an old last_sync_error from a transient failure.
4. Run a GitHub source synchronization cycle.
5. Observe that the result is successful with skipped=True.
6. Inspect the source metadata: last_synced_at, last_sync_status, and last_sync_error are unchanged.
7. Run the background cycle again. The source is still considered stale and GitHub is queried again.

Expected Behavior

A successful same-SHA check should persist a fresh synchronization state:

• refresh last_synced_at;
• set last_sync_status to success;
• clear last_sync_error;
• retain the current last_synced_sha;
• return skipped=True without downloading or re-indexing files.

The source should then remain fresh until the next 24-hour synchronization window.

Related Module

Knowledge Base Management

Configuration Used

A knowledge base with a GitHub source whose last_synced_sha matches the current remote branch SHA and whose last_synced_at is older than the 24-hour synchronization interval.

Logs and screenshots

No runtime log is required to reproduce this issue. The behavior follows directly from the same-SHA early return in deeptutor/services/github_source/sync.py.

Additional Information

• DeepTutor Version: latest main at commit 897fce5
• Operating System: platform-independent
• Python Version: not environment-specific
• Node.js Version: not applicable
• Browser: not applicable
• Related Issues: none found

Relevant code:

• deeptutor/services/github_source/sync.py: same-SHA early return
• deeptutor/services/github_source/sync_service.py: hourly checks and stale filtering
• deeptutor/services/base_sync.py: last_synced_at-based staleness
• deeptutor/services/web_source/sync.py: persists success state after unchanged crawls