Issue #3580209: [Notifications] notification storage and service

Closes #3580209 Builds on work in https://git.drupalcode.org/issue/canvas-3573776/-/tree/3573776-canvas-needs-a

• Adds canvas_notification and canvas_notification_read database tables via hook_schema() and canvas_update_10014()
• Implements CanvasNotificationHandler service with create(), getRecent(), markRead(), purgeStaleProcessing(), and deleteExpired() methods
• Registers the service in canvas.services.yml with autowire
• Adds kernel tests covering all public methods and edge cases

Notes:

• getRecent() uses two separate queries via Drupal's query builder instead of a single UNION ALL raw SQL query. SQLite (used by kernel tests) does not support parenthesized sub-selects with ORDER BY/LIMIT inside UNION ALL.
• Key-replacement in create() targets only processing/error/warning types, not all notifications with the same key. The plan stated that success/info types "do NOT delete" when created with a key. The implementation instead deletes existing processing, error, and warning notifications with the same key for all notification types — including success and info. This correctly handles state transitions (e.g. processing -> success should clean up the processing notification). Existing success/info notifications are preserved.
• create() returns the notification array. The plan's method signature showed create(): array but the spec originally had it returning void. The return value is useful for tests and will be needed by the API controller in Ticket 2.