Upgrade runtime and add containerized local development

[mkarson1997]

Goal

Modernize the backend case study and make it reproducible on a clean machine.

Completed

PR #4 was merged as commit 3b924463060b8959e9c170e7f8247919d1664a53.

• Upgrade all projects from .NET 6 to .NET 10 LTS.
• Upgrade EF Core packages to 10.0.11.
• Pin the repository to the .NET 10 SDK toolchain with global.json.
• Add a multi-stage Dockerfile for the API.
• Add Docker Compose for API + SQL Server.
• Require SQL Server password through ignored .env; commit only .env.example.
• Pass the database connection through environment configuration in Compose.
• Add explicit opt-in migration-on-start configuration for the local Compose environment.
• Add /health/live.
• Add DB-backed /health/ready returning 503 when SQL Server is unavailable.
• Restrict the Hangfire dashboard to Development.
• Rewrite README Quick Start around docker compose up --build.
• Update GitHub Actions to .NET 10 restore/build/tests.
• Validate Docker Compose configuration in CI.
• Build the production-style API container image in CI.
• PR CI passed with 0 build warnings and 0 build errors.

Remaining runtime smoke

• Run the complete SQL Server + API Compose stack on a Docker-capable machine and verify both /health/live and /health/ready end-to-end.

Acceptance criteria

• docker compose up --build starts the complete development stack end-to-end.
• Database initialization/migrations are documented.
• CI builds the upgraded target and container image.
• README quick-start uses the reproducible container path.

The engineering migration is merged. This issue stays open only for the full two-container runtime smoke so the repository does not claim an end-to-end result that CI has not executed.