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/Controllers/EmailController.cs

@@ -0,0 +1,24 @@
+using EmailApi.Models.Requests;
+using EmailApi.Services;
+using Microsoft.AspNetCore.Mvc;
+using Swashbuckle.AspNetCore.Annotations;
+
+namespace EmailApi.Controllers;
+
+[ApiController]
+[Route("api/email")]
+public sealed class EmailController : ControllerBase
+{
+    private readonly SmtpEmailDispatcher _dispatcher;
+
+    public EmailController(SmtpEmailDispatcher dispatcher) => _dispatcher = dispatcher;
+
+    [HttpPost("send")]
+    [SwaggerOperation(Summary = "Send an HTML email via SMTP")]
+    [ProducesResponseType(StatusCodes.Status204NoContent)]
+    public async Task<IActionResult> Send([FromBody] SendEmailRequest request, CancellationToken ct)
+    {
+        await _dispatcher.SendAsync(request, ct);
+        return NoContent();
+    }
+}