fix: make notification repository user-scoped - #626

What?

Make the notification repository interface user-scoped and remove the unused WP\Notifications\Notification interface.

Why?

The repository contract cannot currently query or update notifications for a specific user, or express message fan-out to multiple recipients. The unused notification interface also references types that do not exist.

Fixes #612.

How?

• Use the existing Model\Message and Model\Notification types.
• Add user-scoped lookup, count, dismissal, display, and removal methods.
• Add message fan-out and orphan-purge contracts.
• Document supported query arguments and return shapes.
• Pass the user ID through the date-range helpers.
• Update the database stubs and load the Order interface during bootstrap.

Database persistence remains part of #42.

Testing Instructions

1. Install dependencies with npm ci and composer install.
2. Start the test environment with npx wp-env start.
3. Run npm run test:php.
4. Confirm the new repository test passes, verifying that find_latest() forwards the user ID, date range, sort order, pagination, and offset.
5. Run composer lint.

Use of AI Tools

AI assistance: Yes
Tool: OpenAI Codex