fix(backend): normalize naive datetime from Redis in knowledge_manager (#3618)

[mrveiss]

Closes #3618

Summary

• Normalize naive datetimes read from Redis by adding UTC tzinfo when tzinfo is None
• Prevents TypeError in naive/aware comparison after fix(backend): replace naive datetime.now() with UTC-aware calls (#3613) #3615 UTC migration

Root cause

PR #3615 migrated datetime.now() to UTC-aware, but pre-migration Redis data stored naive datetime strings. fromisoformat() returns naive datetimes which raise TypeError on comparison with UTC-aware datetimes. The surrounding except Exception: return False caught it silently, treating all pre-migration man page cache entries as expired.

Fix

Two lines added in _is_man_page_recent() after fromisoformat():

if last_updated.tzinfo is None:
    last_updated = last_updated.replace(tzinfo=timezone.utc)

This is the standard normalization pattern established in the #3615 migration (also applied in auth_middleware.py and agent_client.py).

[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!