feat(cron): deduplicate and protect scheduled executions

[JAVA-LW]

Summary

• add task-level queue protection for scheduled cron executions
• deduplicate scheduled occurrences with a database-backed (job_id, scheduled_at) claim
• add renewable run leases, heartbeat renewal, retry preservation, and restart recovery
• claim occurrences before conversation preparation/creation
• keep schedule calculation on the theoretical occurrence grid

Closes #600

Why

Conversation busy state alone cannot prevent duplicate callbacks from creating two independent conversations. The occurrence claim must happen at the task/database level before any conversation is materialized.

Implementation

• add queue_enabled to cron jobs
• add cron_job_runs with a unique occurrence key and active-run index
• use BEGIN IMMEDIATE for atomic cross-process claims
• record duplicate, running, retrying, skipped, success, and error outcomes
• use owner IDs and expiring leases for crash recovery
• preserve scheduled_at through retries and scheduler callbacks
• fence stale owners from updating job state after losing their lease

Testing

• RUSTUP_TOOLCHAIN=stable cargo fmt --all -- --check
• RUSTUP_TOOLCHAIN=stable cargo check -p aionui-cron -p aionui-db -p aionui-app
• RUSTUP_TOOLCHAIN=stable cargo test -p aionui-cron -p aionui-db -p aionui-api-types
• RUSTUP_TOOLCHAIN=stable cargo test -p aionui-app --test cron_e2e

All checks pass on the latest upstream main.

Related UI

The corresponding queue-protection UI and visual schedule builder are in iOfficeAI/AionUi#3552.