feat: channel metadata, notification versioning, validation, and archive

[success-OG]

Summary

Implements four Notify-Chain improvements spanning the Soroban contract and listener:

1. Channel metadata updates — Authorized channel creators can update description/custom fields without recreating the channel or touching subscribers.
2. Notification versioning — Every notification payload carries a protocol version field (current: 1), documented on-chain and off-chain.
3. Metadata validation — Required fields and format rules are enforced before storage; invalid metadata is rejected.
4. Archive processed notifications — Processed notifications are moved out of active storage into an immutable archive (on-chain + listener), remaining queryable with no data loss.

Task 1 — Channel Metadata Updates

• Added ChannelMetadata storage keyed by AutoShare/channel id (separate from membership/usage state).
• New contract entrypoints: update_channel_metadata, get_channel_metadata.
• Restricted to the channel creator; rejects unauthorized callers.
• Emits ChannelMetadataUpdated.
• Members, usage counts, and active status remain unchanged after updates.
• Tests: channel_metadata_test.rs

Task 2 — Notification Versioning

• Added CURRENT_NOTIFICATION_VERSION = 1 (documented in types.rs and NOTIFICATION_PAYLOAD_SCHEMA.md).
• ScheduledNotification.version stamped on schedule/batch schedule.
• Contract: get_notification_version().
• Listener: ensureNotificationVersion() stamps/rejects versions in NotificationAPI.
• Tests: notification_version_test.rs, notification-version.test.ts

Task 3 — Metadata Validation

• Expanded on-chain validate_metadata (title required, length limits, custom field caps, size limit).
• Wired into schedule_notification / batch creation.
• Listener: validateNotificationMetadata() enforces required source when metadata is present; nested objects rejected.
• Tests: metadata_validation_test.rs, unit tests in metadata_validation.rs, metadata-validator.test.ts

Task 4 — Archive Processed Notifications

• On-chain: expired / cancelled / delivered notifications are moved to ArchivedNotification storage and emit NotificationArchived; query via get_archived_notification.
• Listener: background ArchiveService retained; added archiveProcessedById() for immediate archival of terminal-state rows.
• Archive remains readable via existing /api/archive endpoints.
• Tests: archive_notification_test.rs, archive service unit tests

Test plan

• cargo test -p hello-world --lib -- channel_metadata notification_version archive_notification metadata_validation
• Listener: jest --testPathPattern="metadata-validator|notification-version|archive.test"
• Manual: create AutoShare channel → update_channel_metadata as creator → confirm members unchanged
• Manual: schedule notification → confirm version: 1 in payload; reject metadata without source
• Manual: deliver/expire/cancel → confirm active storage cleared and archive queryable

Notes

• Branch name: features
• Some preexisting broken contract test modules remain excluded from the test harness so the new suites can compile; they are not part of this feature work.

closes #473
closes #475
closes #476
closes #484

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