Stabilize startup policy and add LibreChat stack

[lraigosov]

Summary

• refine restart policies so only essential baseline services auto-start
• add dedicated LibreChat stack and related configuration files
• update operational documentation and structure references
• preserve manual-start behavior for non-baseline services

Validation performed

• validated compose rendering for PostgreSQL stack
• validated compose rendering for Portainer stack
• validated compose rendering for LibreChat stack with env file
• confirmed repository branch state and clean working tree before PR

Review notes

• focus review on restart policy intent: only portainer and postgres_server should remain auto-start
• verify new LibreChat files align with existing compose/env/config conventions
• no branch deletion requested after merge