SS-1930 Add automatic cleanup for idle Postgres sessions

[hamzaimran08]

Description

This PR relates to SS-1930.

Adds PostgreSQL session timeout configuration for Studio database connections so idle sessions are cleaned up automatically. The defaults are:

• POSTGRES_IDLE_SESSION_TIMEOUT=30m
• POSTGRES_IDLE_IN_TRANSACTION_SESSION_TIMEOUT=10m

The Django database configuration now passes these values through PostgreSQL connection options and enables CONN_HEALTH_CHECKS, so pooled connections are checked before reuse if PostgreSQL has closed an idle connection. The new environment variables are also documented in .env.template.

Checklist

• This pull request is against develop branch (not applicable for hotfixes)
• I have included a link to the issue on GitHub or JIRA (if any)
• I have included migration files (if there are changes to the model classes)
• I have added or updated unit and end2end tests or a manual test case to complement my changes
• I have ran unit and end2end tests
• I have updated the related documentation (if necessary)
• I have added a reviewer for this pull request
• I have added myself as an author for this pull request
• In the case I have modified settings.py, then I have also updated the studio-settings-configmap.yaml file in serve-charts
• In case your changes are large enough, did you deploy your changes to develop instance?

[j-awada]

Works as expected locally and LGTM.