fix: use ClaimedRewards storage for staking-payouts claimed status post-AHM

[eugypalu]

Summary

Fix accounts/{address}/staking-payouts returning already-claimed payouts as unclaimed on Asset Hub (post-AHM).

Root cause: After the Asset Hub Migration, pallet-staking-async records claimed rewards in the Staking.ClaimedRewards(era, validator) storage map. The legacy legacyClaimedRewards field in the staking ledger is no longer updated. Sidecar was only reading the legacy field, so it always saw claimed: false for post-AHM eras.

Fix: In deriveEraPayouts, check ClaimedRewards storage first, comparing claimed pages against pageCount from ErasStakersOverview. Fall back to the legacy ledger fields only when the new storage doesn't exist in the runtime (pre-AHM). This matches the approach used by polkadot-rest-api and already present in sidecar's own AccountsStakingInfoService.

Changes:

• deriveEraPayouts made async, receives historicApi (required) to query storage at the correct block
• All three call sites updated (fetchAccountStakingPayout, fetchErasFromRelayChain, fetchErasFromAssetHub)
• Added tests covering the new ClaimedRewards storage path

Before (sidecar): era 2101 returned with claimed: false and payout data, even though the reward was already claimed.
After (sidecar): era 2101 correctly returns "payouts": [] with unclaimedOnly=true, matching polkadot-rest-api.

[TarikGul]

Looks good 👍 Thanks for the fix.