feat(backend): add retry queue for failed recordPaymentOnChain calls

[leocagli]

Closes #129

Summary

recordPaymentOnChain in both weather and search handlers was called fire-and-forget with .catch(warn) — a transient RPC failure silently dropped agent score updates, causing score drift between on-chain state and reality with no recovery path.

• backend/src/lib/paymentRetryQueue.js (new): in-memory queue with exponential backoff (5s → 10s → 20s → 40s → 80s, max 5 attempts). A 5s setInterval worker (unref'd so it does not block process exit) processes due entries.
• services.js: weather and search handlers now call enqueuePaymentRecord(agentAddress, fn) in their catch block instead of silently discarding the failure.
• After MAX_ATTEMPTS, the entry is dropped and logged at error level with full context for operator alerting.

The queue is in-process only — lost on restart. This handles the common transient failure case (RPC hiccup, network timeout) without SQLite/file persistence complexity.

Test plan

• Mock recordPaymentOnChain to fail 3 times then succeed — verify the retry queue succeeds on the 4th attempt
• Mock it to always fail — verify it is dropped after 5 attempts with an error log
• Normal operation: successful first call is not enqueued

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📥 Commits

Reviewing files that changed from the base of the PR and between 6fb6da9 and 8311b11.

📒 Files selected for processing (2)

• backend/src/lib/paymentRetryQueue.js
• backend/src/routes/services.js

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🧪 Generate unit tests (beta)

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