Store and download last applied configuration snapshot

[somiljain2006]

Fixes #1023

Allows downloading the last applied configuration

• Saves the full exported configuration to 'casc-last-applied.yaml' upon successful reload.
• Adds a "Download last applied configuration" button to the JCasC management UI.
• Implemented a REST endpoint with SYSTEM_READ permission checks.
• Ensures failed configuration attempts do not overwrite the last known good state.
• Includes JUnit tests for the new endpoint and file persistence.

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