BettaPay-Contract Issues Close #699, #700, #701, #702

[orebams]

Summary

Closes #699
Closes #700
Closes #701
Closes #702

• Makes get_payment_reference and get_payments public so indexers and composing contracts can verify known 32-byte payment references without merchant signing authority.
• Documents payment references as capability-style lookup values; indexers can discover records through payment_stored events and verify them through read endpoints or raw state.
• Updates timelocked execute to accept and authenticate an explicit executor, then emits that address in scheduled-operation events instead of the primary admin.
• Moves governance FeeConfig into instance storage to prevent independent persistent-storage TTL expiry.
• Moves settlement DefaultRule into instance storage and removes its explicit persistent TTL extension logic.

Breaking change

execute now requires an executor: Address argument.

Validation

• Ran git diff --check.

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