[add]: Database connection and migrations

app/models/customer.py

@@ -0,0 +1,12 @@
+from datetime import datetime
+from app import db
+
+class Customer(db.Model):
+    id = db.Column(db.String(50), primary_key=True) 
+    msisdn = db.Column(db.String(20), unique=True, nullable=False)
+    country_code = db.Column(db.String(3), nullable=False)
+    created_at = db.Column(db.DateTime, default=datetime.utcnow)
+    updated_at = db.Column(db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+    def __repr__(self):
+        return f'<Customer {self.id}>'