feat(backend): Add running balance and introduce atomicity on transaction creation

[majdyz]

Getting balance can be slow without the balance snapshotting.
The scope of this change is the introduction of runningBalance column on the transaction table.
The change scope includes ensuring the running balance calculation is protected by a transaction & last-row locking.

Changes 🏗️

• Added runningBalance column on CreditTransaction table.
• Added credit model with normal balance calculation UserCredit & its default monthly version BetaUserCredit.

Checklist 📋

For code changes:

• I have clearly listed my changes in the PR description
• I have made a test plan
• I have tested my changes according to the test plan:
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Example test plan

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For configuration changes:

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Examples of configuration changes

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[qodo-merge-pro]

PR Reviewer Guide 🔍

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⏱️ Estimated effort to review: 4 🔵🔵🔵🔵⚪

🧪 PR contains tests

🔒 Security concerns SQL Injection: The SQL query in _add_transaction() uses string formatting with user input (user_id). While the user_id likely comes from a trusted source, using parameterized queries would be safer to prevent any potential SQL injection vulnerabilities.

⚡ Recommended focus areas for review Race Condition The transaction locking mechanism in _add_transaction() may not fully prevent race conditions since it only locks the latest balance snapshot. Multiple concurrent transactions could still potentially read/write balances in an inconsistent way. async with db.transaction() as tx: # Acquire lock on latest balance snapshot await tx.execute_raw( f"""SELECT * FROM platform."CreditTransaction" WHERE "userId" = '{user_id}' AND "isActive" = true ORDER BY "createdAt" DESC FOR UPDATE """ ) Error Handling The _get_balance() method does not handle database errors or invalid data scenarios. Missing error handling could lead to unexpected behavior or crashes. async def _get_balance(user_id: str, end_time: datetime = datetime.max) -> int: # Find the latest captured balance snapshot. snapshot = await CreditTransaction.prisma().find_first( where={ "userId": user_id, "createdAt": {"lt": end_time}, "isActive": True, "runningBalance": {"not": None}, # type: ignore }, order={"createdAt": "desc"}, ) snapshot_balance = (snapshot.runningBalance or 0) if snapshot else 0 begin_time = snapshot.createdAt if snapshot else datetime.min # Sum all transactions after the last captured balance snapshot. transactions = await CreditTransaction.prisma().group_by( by=["userId"], sum={"amount": True}, where={ "userId": user_id, "createdAt": {"gt": begin_time, "lt": end_time}, "isActive": True, }, ) transaction_balance = ( transactions[0].get("_sum", {}).get("amount", 0) if transactions else 0 ) return snapshot_balance + transaction_balance

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