Refactor backend toward Clean Architecture

[patryk-pszeniczny]

Summary

• refactor the backend toward Clean/Onion Architecture with clearer Domain, Application, Infrastructure, and API boundaries
• replace Minimal API endpoint modules with controllers and dedicated command/query handlers
• add application validation behavior, typed application exceptions, and centralized API exception handling
• extract repository and notification interfaces and move infrastructure-specific implementations outward
• simplify domain entities and add focused unit tests for domain and application behavior

Verification

• dotnet test Messager.slnx --no-restore
• 28 tests passed, 0 failed, 0 skipped

Notes

This PR preserves the existing API use cases while restructuring the backend for clearer responsibilities and testability.