Server-side filtering backup for client-side session picker search.
Case-insensitive substring match on displayName, label, subject,
sessionId, and key.
Closes#1161
Enable meaningful session titles via priority-based derivation:
1. displayName (user-set)
2. subject (group name)
3. First user message (truncated to 60 chars)
4. sessionId prefix + date fallback
Opt-in via includeDerivedTitles param to avoid perf impact on
regular listing. Reads only first 10 lines of transcript files.
Closes#1161
