feat: add environment variable validation on service startup

[Mrsandeep27]

Summary

Closes #1201

• Created reusable EnvironmentValidator component in common_service that validates required env vars on ApplicationStartedEvent
• Logs a clear error listing all missing variables and fails fast with IllegalStateException
• Common vars (spring.datasource.url/username/password) validated by default for all services
• Added EnvironmentValidationConfig in auth_service as an example of registering service-specific required vars
• Other services can follow the same pattern by injecting EnvironmentValidator and calling addRequiredVariables()

Test plan

• Start auth_service with missing datasource config and verify clear error message
• Start auth_service with all vars set and verify "validation passed" log
• Verify other services also get common variable validation automatically

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