Add IMessageChannel and Line REST implementation

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

API/ROLAC.API/Services/Notifications/IMessageChannel.cs

@@ -0,0 +1,12 @@
+namespace ROLAC.API.Services.Notifications;
+
+/// <summary>Result of one Line REST call.</summary>
+public sealed record MessageSendResult(bool Success, string? Error);
+
+/// <summary>Abstraction over a chat channel's send/reply (Line today; future channels later).</summary>
+public interface IMessageChannel
+{
+    Task<MessageSendResult> PushToUserAsync(string externalId, string text, CancellationToken ct = default);
+    Task<MessageSendResult> PushToGroupAsync(string externalId, string text, CancellationToken ct = default);
+    Task<MessageSendResult> ReplyAsync(string replyToken, string text, CancellationToken ct = default);
+}

API/ROLAC.API/Services/Notifications/LineMessageChannel.cs

@@ -0,0 +1,52 @@
+using System.Net.Http.Headers;
+using System.Net.Http.Json;
+using Microsoft.Extensions.Options;
+
+namespace ROLAC.API.Services.Notifications;
+
+/// <summary>Sends text messages and replies via the Line Messaging API REST endpoints.</summary>
+public sealed class LineMessageChannel : IMessageChannel
+{
+    private const string PushUrl  = "https://api.line.me/v2/bot/message/push";
+    private const string ReplyUrl = "https://api.line.me/v2/bot/message/reply";
+
+    private readonly HttpClient _http;
+    private readonly LineOptions _options;
+
+    public LineMessageChannel(HttpClient http, IOptions<LineOptions> options)
+    {
+        _http = http;
+        _options = options.Value;
+    }
+
+    public Task<MessageSendResult> PushToUserAsync(string externalId, string text, CancellationToken ct = default)
+        => PostAsync(PushUrl, new { to = externalId, messages = new[] { new { type = "text", text } } }, ct);
+
+    public Task<MessageSendResult> PushToGroupAsync(string externalId, string text, CancellationToken ct = default)
+        => PostAsync(PushUrl, new { to = externalId, messages = new[] { new { type = "text", text } } }, ct);
+
+    public Task<MessageSendResult> ReplyAsync(string replyToken, string text, CancellationToken ct = default)
+        => PostAsync(ReplyUrl, new { replyToken, messages = new[] { new { type = "text", text } } }, ct);
+
+    private async Task<MessageSendResult> PostAsync(string url, object payload, CancellationToken ct)
+    {
+        try
+        {
+            using var request = new HttpRequestMessage(HttpMethod.Post, url)
+            {
+                Content = JsonContent.Create(payload),
+            };
+            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", _options.ChannelAccessToken);
+
+            using var response = await _http.SendAsync(request, ct);
+            if (response.IsSuccessStatusCode) return new MessageSendResult(true, null);
+
+            var body = await response.Content.ReadAsStringAsync(ct);
+            return new MessageSendResult(false, $"{(int)response.StatusCode}: {body}");
+        }
+        catch (Exception ex)
+        {
+            return new MessageSendResult(false, ex.Message);
+        }
+    }
+}