feat(batch_claim): settle batch claims with actual token payouts (#1198)

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Closes #1198

Summary

Wires real reward-token payouts into CalloraBatchClaim:

1. Adds StorageKey::RewardToken, init_with_token(env, admin, reward_token), set_reward_token(env, caller, reward_token), and get_reward_token(env).
2. Validates the entire batch upfront in batch_claim and verifies token::Client::new(&env, &token_addr).balance(&contract_addr) >= total_claimed before mutating any state (returning BatchClaimError::InsufficientBalance if underfunded).
3. Transfers each claimant's pending_amount after marking ClaimConsumed and record.settled = true, and includes the reward token in claims_settled events.
4. Updates contracts/batch_claim/docs/storage.md to match the contract's StorageKey layout.

Acceptance Criteria Mapping

• Each claimant receives exactly their pending amount: Verified in test_batch_claim_transfers_reward_tokens_to_each_claimant.
• Insufficient contract balance fails before any state change: Checked upfront before the settlement write loop; verified in test_batch_claim_insufficient_contract_balance_fails_without_state_change.
• Events include token: init_with_token and claims_settled include the configured reward_token address.
• Storage doc updated: Updated contracts/batch_claim/docs/storage.md.

Validation

• cargo test -p callora-batch-claim

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Payout Wallet (Base Mainnet USDC / EVM): 0x3806cd592a508d1992Ed007C2F53de251C2E7452