perf: Batch Stellar Horizon balance queries instead of sequential per-token calls

[YaronZaki]

Problem Statement

StellarClient.get_token_balances() in blockchain_call.py:94-114 iterates over tokens and calls get_balance() sequentially for each token. This results in N HTTP requests to Horizon for N tokens when a single account query would return all balances.

Evidence

# quantara/web_app/contract_tools/blockchain_call.py:94-114
async def get_token_balances(self, holder_address: str) -> dict[str, str]:
    balances: dict[str, str] = {}
    for token in TokenParams.tokens():  # Sequential per-token calls!
        bal = await self.get_balance(
            asset_code=token.asset_code,
            holder_address=holder_address,
        )
        balances[token.name] = bal
    return balances

Meanwhile, get_balance() already fetches the full account from Horizon (/accounts/{holder_address}) but then discards all balances except the requested one.

Impact

Medium — unnecessary latency. N sequential HTTP round-trips where 1 would work. Page load time scales linearly with number of supported tokens. Currently 3 tokens = 3 sequential calls; adding more tokens worsens the problem.

Proposed Solution

Refactor get_token_balances() to make a single Horizon /accounts/{holder_address} call, extract all balances from the response, and filter to supported tokens. This reduces N sequential calls to 1.

Acceptance Criteria

• get_token_balances() makes single Horizon call per invocation
• All supported token balances extracted from single account response
• Same return type preserved: dict[str, str]
• Error handling preserved (returns empty balances on failure)
• All existing tests pass

File Map

• quantara/web_app/contract_tools/blockchain_call.py:94-114 — refactor to single call

Dependencies

• Related: REPO-018 (caching layer complements batching)

Testing Strategy

• Unit: Test with mocked account response containing multiple balances
• Integration: Compare results of old sequential vs. new batched approach for same account

Security Considerations

No security impact. Pure performance optimization.

Definition of Done

• Code implemented and peer-reviewed
• Tests written and passing
• PR linked and merged

---

Labels: performance
Priority: Medium
Difficulty: Intermediate
Estimated Effort: 3h

[Nemenwq]

Hi maintainer, I have reviewed the issue description and will like to implement Batch Stellar Horizon balance queries instead of sequential per-token calls. ETA: 4hours, thank you

[grantfox-oss]

🦊 GrantFox — @Nemenwq has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #60)
2. Your PR will be reviewed by the Quantarq maintainers

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Nemenwq's PR #162 was approved and merged by @YaronZaki.

🏆 @Nemenwq: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (366 total points). Track your full progress on GrantFox.

👏 Great work, @Nemenwq! Keep contributing to Quantarq.