feat: add RabbitMQ worker, contracts, usage UI in channels screen

- Add HrynCo.NotificationService.Contracts project with SendEmailMessage and NotificationResultMessage
- Add SendEmailConsumer (RabbitMQ worker) with reply-to pattern via CorrelationContext.ReplyTo
- Add SendEmailHandler owning SMTP send + usage increment as business logic
- Add GetChannelUsageSummaryHandler with single DB query via navigation property
- Merge usage stats inline into channels list (daily/monthly with progress bars)
- Refactor AdminChannelsController.Index to use GetChannelUsageSummaryQuery
- Add RabbitMQ service to docker-compose files
- Remove dead AdminChannelUsageController, ChannelUsageViewModel, ChannelUsageSummary

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Contracts/HrynCo.NotificationService.Contracts.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="HrynCo.RabbitMq" />
+  </ItemGroup>
+
+</Project>

HrynCo.NotificationService.Contracts/Messages/NotificationResultData.cs

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+namespace HrynCo.NotificationService.Contracts.Messages;
+
+public record NotificationResultData
+{
+    public required string ServiceName { get; init; }
+    public required string RecipientEmail { get; init; }
+    public required string TemplateKey { get; init; }
+    public required DateTimeOffset Timestamp { get; init; }
+
+    /// <summary>Null when delivery succeeded; contains error details on failure.</summary>
+    public string? ErrorMessage { get; init; }
+
+    public bool IsSuccess => ErrorMessage is null;
+}

HrynCo.NotificationService.Contracts/Messages/NotificationResultMessage.cs

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+namespace HrynCo.NotificationService.Contracts.Messages;
+
+using Hrynco.RabbitMq;
+
+public record NotificationResultMessage : IRabbitMqMessage<NotificationResultData>
+{
+    public CorrelationContext CorrelationContext { get; set; } = null!;
+    public NotificationResultData Data { get; set; } = null!;
+}

HrynCo.NotificationService.Contracts/Messages/SendEmailMessage.cs

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+namespace HrynCo.NotificationService.Contracts.Messages;
+
+using Hrynco.RabbitMq;
+
+public record SendEmailMessage : IRabbitMqMessage<SendEmailMessageData>
+{
+    public CorrelationContext CorrelationContext { get; set; } = default!;
+    public SendEmailMessageData Data { get; set; } = default!;
+}

HrynCo.NotificationService.Contracts/Messages/SendEmailMessageData.cs

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+namespace HrynCo.NotificationService.Contracts.Messages;
+
+public record SendEmailMessageData
+{
+    public required string ServiceName { get; init; }
+    public required string TemplateKey { get; init; }
+    public required string RecipientEmail { get; init; }
+    public required string RecipientName { get; init; }
+    public required IReadOnlyDictionary<string, string> Variables { get; init; }
+    public string? LanguageCode { get; init; }
+}