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/Clients/IEmailApiClient.cs

@@ -0,0 +1,10 @@
+using EmailApi.Models.Requests;
+using Refit;
+
+namespace EmailApi.Models.Clients;
+
+public interface IEmailApiClient
+{
+    [Post("/api/email/send")]
+    Task SendAsync(SendEmailRequest request, CancellationToken ct = default);
+}

Apis/email-api-models/Requests/SendEmailRequest.cs

@@ -0,0 +1,10 @@
+namespace EmailApi.Models.Requests;
+
+public sealed class SendEmailRequest
+{
+    public required List<string> To { get; init; }
+    public string? ReplyTo { get; init; }
+    public required string Subject { get; init; }
+    public required string HtmlBody { get; init; }
+    public string? AttachmentPath { get; init; }
+}

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; } = "";
+}

Apis/email-api-models/email-api-models.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <AssemblyName>email-api-models</AssemblyName>
+    <RootNamespace>EmailApi.Models</RootNamespace>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Refit.HttpClientFactory" />
+  </ItemGroup>
+</Project>