Implement on-chain notification expiration (Closes #128)

[giftexceed]

Summary

Implements configurable-duration notification expiration for notifications stored on-chain (issue #128).

Feature

• schedule_notification(id, creator, ttl_seconds) — stores a ScheduledNotification { id, creator, created_at, expires_at } where expires_at = created_at + ttl_seconds. Rejects a zero or overflowing duration (InvalidExpirationDuration) and a duplicate id (AlreadyExists); emits NotificationScheduled.
• get_notification(id) / is_notification_expired(id) — read a notification's details / validity. The expiry boundary is inclusive (expired once now >= expires_at).
• expire_notification(id) — permissionless finalization of an elapsed notification (e.g. by an off-chain keeper): reaps storage and emits NotificationExpired. A not-yet-elapsed notification is rejected with NotificationNotExpired.
• cancel_notification(id, caller) — now treats an expired tracked notification as invalid (NotificationExpired) and reaps a valid one on cancellation, while still accepting untracked ids for backward compatibility.

New ScheduledNotification type; new errors NotificationExpired, InvalidExpirationDuration, NotificationNotExpired; new events NotificationScheduled / NotificationExpired (category Notification), following the existing trailing category/priority topic convention.

Tests

15 new contract tests in tests/expiration_test.rs cover: storage of created/expiry timestamps, the expiry boundary (before / at / after), NotificationScheduled and NotificationExpired event shapes (topics + data), expired notifications being uncancellable, finalized expiry reaping storage, duration validation, duplicate/unknown handling, pause gating, and cancellation (valid, expired, untracked, paused).

Heads-up: this PR also repairs a broken main

The contract crate did not compile on main. Prior merges around the notification-priority-levels work were resolved by keeping both sides of each conflict, leaving duplicated enum definitions, struct fields, imports, and test functions in events.rs, autoshare_logic.rs, and notification_test.rs (e.g. two NotificationPriority enums; 18 priority fields where 9 were expected; duplicated priority_of test helpers and malformed assert_eq! calls).

Making #128's tests pass required restoring a compiling crate. I resolved the duplications to the coherent Low/Medium/High/Critical priority scheme (recovered from the last clean commit, 33c5764), re-added the ScheduledNotificationCancelled event/handler cleanly, and aligned AutoShareDetails.priority to the same scheme.

Verification

cargo test  →  119 passed; 0 failed
cargo fmt --check  →  clean
cargo build  →  0 warnings

Closes #128

[Abd-Standard]

please fix CI and resolve conflicts