fix: fallback title from preview when session has no explicit title

SQLite path was returning null title for sessions without an explicit
title, while the CLI path derives it from the first user message.
Now uses the preview (first user message content) as title fallback,
matching the original CLI behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/server/sessions-db.test.ts

@@ -120,6 +120,6 @@ describe('session DB summaries', () => {
     expect(allMock).toHaveBeenCalledWith('telegram', 2)
     expect(rows[0].last_active).toBe(1710000100)
     expect(rows[0].source).toBe('telegram')
-    expect(rows[0].title).toBeNull()
+    expect(rows[0].title).toBe('preview text')
   })
 })