Adds transactional outbox and email verification

Implements outbox pattern for reliable message delivery
Adds email verification flow with Postmark integration
Enhances account registration with secure token generation

Introduces background processing for asynchronous email sending
Implements database-level notification mechanism for message processing

Core/Outbox/OutboxService.cs

@@ -0,0 +1,104 @@
+#nullable enable
+
+using System.Data;
+using System.Text.Json;
+using Insight.Database;
+using PlanTempus.Core.Database;
+
+namespace PlanTempus.Core.Outbox;
+
+public class OutboxService(IDatabaseOperations databaseOperations) : IOutboxService
+{
+	public async Task EnqueueAsync(string type, object payload, IDbConnection? connection = null, IDbTransaction? transaction = null)
+	{
+		var sql = @"
+			INSERT INTO system.outbox (type, payload)
+			VALUES (@Type, @Payload::jsonb)";
+
+		var parameters = new
+		{
+			Type = type,
+			Payload = JsonSerializer.Serialize(payload)
+		};
+
+		if (connection != null)
+		{
+			await connection.ExecuteSqlAsync(sql, parameters);
+		}
+		else
+		{
+			using var db = databaseOperations.CreateScope(nameof(OutboxService));
+			await db.Connection.ExecuteSqlAsync(sql, parameters);
+		}
+	}
+
+	public async Task<List<OutboxMessage>> GetPendingAsync(int batchSize = 10)
+	{
+		using var db = databaseOperations.CreateScope(nameof(OutboxService));
+
+		var sql = @"
+			UPDATE system.outbox
+			SET status = 'processing'
+			WHERE id IN (
+				SELECT id FROM system.outbox
+				WHERE status = 'pending'
+				ORDER BY created_at
+				LIMIT @BatchSize
+				FOR UPDATE SKIP LOCKED
+			)
+			RETURNING id, type, payload, status, created_at, processed_at, retry_count, error_message";
+
+		var results = await db.Connection.QuerySqlAsync<OutboxMessageDto>(sql, new { BatchSize = batchSize });
+
+		return results.Select(r => new OutboxMessage
+		{
+			Id = r.Id,
+			Type = r.Type,
+			Payload = JsonSerializer.Deserialize<JsonElement>(r.Payload),
+			Status = r.Status,
+			CreatedAt = r.CreatedAt,
+			ProcessedAt = r.ProcessedAt,
+			RetryCount = r.RetryCount,
+			ErrorMessage = r.ErrorMessage
+		}).ToList();
+	}
+
+	public async Task MarkAsSentAsync(Guid id)
+	{
+		using var db = databaseOperations.CreateScope(nameof(OutboxService));
+
+		var sql = @"
+			UPDATE system.outbox
+			SET status = 'sent', processed_at = NOW()
+			WHERE id = @Id";
+
+		await db.Connection.ExecuteSqlAsync(sql, new { Id = id });
+	}
+
+	public async Task MarkAsFailedAsync(Guid id, string errorMessage)
+	{
+		using var db = databaseOperations.CreateScope(nameof(OutboxService));
+
+		var sql = @"
+			UPDATE system.outbox
+			SET status = 'failed',
+				processed_at = NOW(),
+				retry_count = retry_count + 1,
+				error_message = @ErrorMessage
+			WHERE id = @Id";
+
+		await db.Connection.ExecuteSqlAsync(sql, new { Id = id, ErrorMessage = errorMessage });
+	}
+
+	private class OutboxMessageDto
+	{
+		public Guid Id { get; set; }
+		public string Type { get; set; } = "";
+		public string Payload { get; set; } = "";
+		public string Status { get; set; } = "";
+		public DateTime CreatedAt { get; set; }
+		public DateTime? ProcessedAt { get; set; }
+		public int RetryCount { get; set; }
+		public string? ErrorMessage { get; set; }
+	}
+}