This PR addresses #7743 by making the public database proxy timeout configurable in the GUI.

Changes:

• Added public_proxy_timeout field to all database models via migration.
• Updated StartDatabaseProxy action to use the configured timeout in Nginx (stream module).
• Default timeout remains 600s (10 minutes) for backward compatibility.
• Users can set it to 0 to disable the timeout or any valid Nginx time value (e.g., 1h).
• Added the configuration field to the General configuration tab for all standalone databases.

/claim #7743