Allow permissionless cranking of due recurring payments

[akprinciple]

closes #449

Summary

This PR implements a permissionless execution model for disburse_recurring, allowing any address to trigger due payouts on active recurring payment schedules.

Under this model, any caller can execute the disbursement crank once a schedule becomes due, provided the contract is not frozen and the schedule remains eligible for settlement. The restrictive owner-only requirement has been removed while retaining all core safety and freeze validations.

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Background & Architectural Trade-offs

The design decision centered on whether recurring disbursement execution should be restricted to contract owners or open as a public keeper crank:

• Benefits: A permissionless crank significantly simplifies off-chain automation (e.g., Gelato, Chainlink Keepers, or custom cron bots) and reduces the risk of missed payouts by allowing third parties or beneficiaries to trigger execution directly.
• Trade-offs: Opening the entrypoint increases surface area for opportunistic execution or front-running disbursement triggers.

Adopting a permissionless pattern balances high automation reliability with robust contract safety by preserving freeze controls and strict eligibility criteria.

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What Changed

• Access Control Refactor: Removed the restrictive require_owner check from the active disburse_recurring entrypoint.
• Preserved Safety Invariants: Retained caller.require_auth() and the require_not_frozen(&env)? state validation.
• Inline Documentation: Documented the automation vs. timing trade-offs directly within the function comments.

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Why

• Keeper-Friendly Automation: Enables third-party bots and decentralized keepers to process due payouts autonomously without privileged keys.
• High Settlement Reliability: Prevents disbursement stalls if the owner account is inactive, unavailable, or experiencing operational delays.
• Strict State Safety: Maintains invariant guarantees that disbursements cannot occur while the contract is frozen.

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