Implement end-to-end service hardening

[UsmanovMahmudkhan]

Summary

Type of Change

• Documentation
• Bug fix
• Feature
• Security hardening
• Refactor
• Test
• Deployment/configuration

Security Impact

Describe authentication, authorization, cryptography, key management, logging, privacy, deployment, or data-handling impact.

For cryptographic/security changes, explain assumptions, failure behavior, downgrade risks, and review needs.

Tests Performed

• mvn test
• flutter test
• Manual testing
• Not applicable

Documentation Updated

• Yes
• No, not needed

Checklist

• I did not commit plaintext secrets, private keys, tokens, credentials, or real user data.
• I avoided exaggerated security claims.
• I updated API, deployment, threat model, or security docs if behavior changed.
• I added or updated tests for behavior changes.
• I considered metadata, logging, and audit impact.

Breaking Changes

List any breaking changes, migrations, environment variable changes, or deployment changes.