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

Database/Core/DDL/SetupOutbox.cs

@@ -0,0 +1,115 @@
+using Insight.Database;
+using System.Data;
+using PlanTempus.Core.Database.ConnectionFactory;
+
+namespace PlanTempus.Database.Core.DDL;
+
+/// <summary>
+/// Sets up the outbox table for reliable message delivery (transactional outbox pattern).
+/// Messages are inserted in the same transaction as the business operation,
+/// then processed asynchronously by a background worker.
+/// </summary>
+public class SetupOutbox(IDbConnectionFactory connectionFactory) : IDbConfigure<SetupOutbox.Command>
+{
+	public class Command
+	{
+		public required string Schema { get; init; }
+	}
+
+	private Command _command;
+
+	public void With(Command command, ConnectionStringParameters parameters = null)
+	{
+		_command = command;
+
+		using var conn = parameters is null ? connectionFactory.Create() : connectionFactory.Create(parameters);
+		using var transaction = conn.OpenWithTransaction();
+
+		try
+		{
+			CreateOutboxTable(conn);
+			CreateOutboxIndexes(conn);
+			CreateNotifyTrigger(conn);
+
+			transaction.Commit();
+		}
+		catch (Exception ex)
+		{
+			transaction.Rollback();
+			throw new InvalidOperationException("Failed to SetupOutbox. Transaction is rolled back", ex);
+		}
+	}
+
+	/// <summary>
+	/// Creates the outbox table for storing pending messages
+	/// </summary>
+	void CreateOutboxTable(IDbConnection db)
+	{
+		var sql = @$"
+			CREATE TABLE IF NOT EXISTS {_command.Schema}.outbox (
+				id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+				type VARCHAR(50) NOT NULL,
+				payload JSONB NOT NULL,
+				status VARCHAR(20) NOT NULL DEFAULT 'pending',
+				created_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+				processed_at TIMESTAMPTZ NULL,
+				retry_count INT NOT NULL DEFAULT 0,
+				error_message TEXT NULL,
+				CONSTRAINT chk_outbox_status CHECK (status IN ('pending', 'processing', 'sent', 'failed'))
+			);
+
+			COMMENT ON TABLE {_command.Schema}.outbox IS 'Transactional outbox for reliable message delivery';
+			COMMENT ON COLUMN {_command.Schema}.outbox.type IS 'Message type (e.g. verification_email, welcome_email)';
+			COMMENT ON COLUMN {_command.Schema}.outbox.payload IS 'JSON payload with message-specific data';
+			COMMENT ON COLUMN {_command.Schema}.outbox.status IS 'pending -> processing -> sent/failed';
+		";
+
+		db.ExecuteSql(sql);
+	}
+
+	/// <summary>
+	/// Creates indexes for efficient polling of pending messages
+	/// </summary>
+	void CreateOutboxIndexes(IDbConnection db)
+	{
+		var sql = @$"
+			CREATE INDEX IF NOT EXISTS idx_outbox_pending
+				ON {_command.Schema}.outbox(created_at)
+				WHERE status = 'pending';
+
+			CREATE INDEX IF NOT EXISTS idx_outbox_failed_retry
+				ON {_command.Schema}.outbox(created_at)
+				WHERE status = 'failed' AND retry_count < 5;
+		";
+
+		db.ExecuteSql(sql);
+	}
+
+	/// <summary>
+	/// Creates a trigger that sends a NOTIFY when new messages are inserted
+	/// </summary>
+	void CreateNotifyTrigger(IDbConnection db)
+	{
+		var sql = @$"
+			CREATE OR REPLACE FUNCTION {_command.Schema}.notify_outbox_insert()
+			RETURNS TRIGGER AS $$
+			BEGIN
+				PERFORM pg_notify('outbox_messages', json_build_object(
+					'id', NEW.id,
+					'type', NEW.type
+				)::text);
+				RETURN NEW;
+			END;
+			$$ LANGUAGE plpgsql;
+
+			DROP TRIGGER IF EXISTS trg_outbox_notify ON {_command.Schema}.outbox;
+
+			CREATE TRIGGER trg_outbox_notify
+				AFTER INSERT ON {_command.Schema}.outbox
+				FOR EACH ROW
+				EXECUTE FUNCTION {_command.Schema}.notify_outbox_insert();
+		";
+
+		db.ExecuteSql(sql);
+	}
+}