feat: add subscription and transaction repositories

- Implemented SubscriptionRepository with methods to get, create, and update subscriptions.
- Added BalanceTXRepository for creating and retrieving balance transactions.
- Introduced a test script for simulating Pally webhook for local testing.

db/models/users.py

@@ -7,6 +7,8 @@ from db.base import Base
 
 if TYPE_CHECKING:
     from db.models.bills import Bill
+    from db.models.subscriptions import Subscription
+    from db.models.transactions import BalanceTransaction
 
 
 class User(Base):
@@ -22,3 +24,9 @@ class User(Base):
         back_populates="user",
         cascade="all, delete",
     )
+    subscription: Mapped["Subscription"] = relationship(
+        "Subscription", back_populates="user", cascade="all, delete-orphan"
+    )
+    balance_transactions: Mapped[list["BalanceTransaction"]] = relationship(
+        "BalanceTransaction", back_populates="user", cascade="all, delete"
+    )