Add complete database schema and ORM models (#4)

internal/models/message_queue.go

@@ -0,0 +1,15 @@
+package models
+
+import (
+	"time"
+)
+
+// MessageQueueEntry represents a pending message delivery for a user.
+type MessageQueueEntry struct {
+	Base
+
+	ID        int64     `json:"id"`
+	UserID    string    `json:"userId"`
+	MessageID string    `json:"messageId"`
+	QueuedAt  time.Time `json:"queuedAt"`
+}