feat: add notification priority levels

[CleanDev-Fix]

Summary

Adds notification priority metadata to the AutoShare contract events and stored group details.

Changes

• Added a NotificationPriority enum with Low, Standard, High, and Critical levels.
• Stored the default group priority on AutoShareDetails.
• Published priority metadata on contract notification events while keeping category as the trailing topic for existing category filters.
• Expanded notification tests to cover persisted priority, emitted priority topics, and event-shape compatibility.

Testing/Verification

• docker run --rm -v "<repo>:/work" -w /work/contract/contracts/hello-world rust:1.86 cargo test passed.
• git diff --check passed.
• cargo fmt -- --check could not be run in this hot-runner shell because host Cargo is unavailable and the tested Rust container lacks cargo-fmt/rustup.

Related issue: closes #130