feat: implement subscription management and billing enhancements

bot/handlers/buy.py

@@ -169,7 +169,12 @@ async def proceed_to_checkout(
     amount = calculate_price(ctx)
     user = await deps.user_repository.get_user_by_id(session, cb.from_user.id)
 
-    if user.balance < amount:
+    credit = await deps.user_service.calculate_plan_credit(
+        session, cb.from_user.id, ctx, deps.rw_sdk
+    )
+    effective_balance = user.balance + credit
+
+    if effective_balance < amount:
         await cb.message.edit_text(
             INSUFFICIENT_FUNDS.format(balance=user.balance, amount=amount), reply_markup=deposit_kb
         )
@@ -190,6 +195,15 @@ async def proceed_to_checkout(
         await cb.message.edit_text(CALLBACK_FALLBACK)
         return
 
+    if credit > 0:
+        await deps.user_repository.increase_user_balance(
+            session,
+            user.id,
+            credit,
+            tx_type=BalanceTxType.REFUND,
+            description="refund for unused subscription days",
+        )
+
     deducted_user = await deps.user_repository.decrease_user_balance(
         session,
         user.id,