Add Docker containerization and local compose stack

[Sheulydsp]

What changed

• Added a multi-stage .NET 10 Dockerfile
• Added .dockerignore
• Added Docker Compose configuration for the API and PostgreSQL
• Added PostgreSQL health checks
• Configured Docker networking between the API and PostgreSQL
• Configured Docker-specific JWT and database settings

Validation

• Docker image builds successfully
• PostgreSQL container starts and becomes healthy
• EF Core migrations execute successfully
• API starts successfully in Docker
• /api/reliability/health returns HTTP 200
• 40/40 automated tests pass