Fix DB issue and change default model to gpt-4o

[ihor-chaban]

Summary

These changes helped me to resolve the issues described in #488

• Fixed questionable conditions in message handling which sometimes triggered wrong handler and likely caused DB issues under some circumstances
• Added default_text_model parameter in models.yml which helps to easily change the default model everywhere and changed the default model to gpt-4o
• Updated docker-compose.yml:
  1. Added tmpfs for /db/configdb to avoid creating a new anonymous empty volume on each run
  2. Removed version as it's deprecated
  3. Changed restart policy from always to unless-stopped to be able to stop containers if necessary without deleting them (e.g. could be useful for debugging DB issues)

I've been testing these changes since the above issue was created and it works fine so far.
No DB issues encountered since then.