feat: capture development email in MailHog via the Aspire sample (#73)

[andregoepel]

Closes #73.

What

• AppHost: adds a MailHog container — SMTP on 1025, web UI on 8025 (open it from the Aspire dashboard to read captured mail). The web app WaitFors it.
• Pre-configured email: the AppHost sets the web app's EmailSender__* config to send through MailHog. Because these come from configuration, not the database, email keeps working after a database reset with zero manual setup on the Email Settings page (the MailService uses the EmailSender section as its bootstrap when no DB record exists).
• SmtpEmailSender auth guard: MailHog accepts mail without authentication and doesn't advertise AUTH, and MailKit throws if AuthenticateAsync is called against such a server. The sender now authenticates only when the server advertises it and a username is configured — so MailHog works, and so do other no-auth relays. Real servers that advertise AUTH behave exactly as before.

Why config, not DB seeding

The issue asks that email survive a database reset without re-setup. Configuration is independent of the (persistent-volume) database, so a wipe/reset can't remove it and there's no re-seed-on-startup logic to maintain. The Email Settings page shows these as the "from configuration" baseline, and an admin can still save DB overrides on top.

Verification

• dotnet build -c Release — 0 errors (AppHost compiles against Aspire 13.4.6; SmtpCapabilities guard compiles).
• dotnet test -c Release — 89/89.
• I did not run the full Aspire end-to-end here (needs Docker + the setup/login flow + sending a test mail to view it in the MailHog UI). The auth guard is correct whether or not MailHog advertises AUTH, and the container/endpoint wiring is standard Aspire; please give it a spin to confirm mail lands in the MailHog UI.

Note

MailHog SMTP/UI use fixed host ports 1025/8025 for a stable local URL. If either conflicts on your machine, tell me and I'll switch to Aspire-assigned dynamic ports (wiring the SMTP endpoint into EmailSender__Server/Port via endpoint references).