feat: add email-api service and email-api-models contract project

New internal service that centralises SMTP email sending.
- email-api-models: SendEmailRequest DTO, IEmailApiClient Refit interface, EmailApiSettings
- email-api: SmtpEmailDispatcher (MailKit), EmailController (POST /api/email/send),
  branded HTML shell wrapper, shared-Files-volume attachment support
- Protected by X-Internal-Api-Key via UseInternalApiKeyProtection()
- No exposed Docker port — internal network only (http://email-api:8080)

Closes #22

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Apis/email-api-models/Settings/EmailApiSettings.cs

@@ -0,0 +1,7 @@
+namespace EmailApi.Models.Settings;
+
+public sealed class EmailApiSettings
+{
+    public string BaseUrl { get; set; } = "";
+    public string InternalApiKey { get; set; } = "";
+}