feat(alerts): add export and import for alert settings

[faisalahammad]

Summary

Adds export and import for Stream alert settings so admins can clone alert rules across WordPress installs instead of reconfiguring each site. A new WP_Stream\Alert_Port class serializes all configured alerts to JSON on export and re-creates them on import.
Fixes #1664

Changes

New: WP_Stream\Alert_Port (classes/class-alert-port.php)

• get_export_data() builds the payload from Alerts::get_alerts(), preserving status, alert_type, and the full alert_meta array verbatim (including third-party keys added via the wp_stream_alerts_save_meta filter). Skips ID, date, author, and title because those are site-local or regenerated on save.
• handle_export() sends a JSON download, gated on Alerts::CAPABILITY and a nonce.
• handle_import() handles the uploaded file via wp_handle_upload (json mime only), then appends each entry through Alert::save().
• import_alerts() skips entries with an unknown alert_type (notifier not active on target) or non-array entries, and resets trigger_author to "any user" when the stored user ID does not exist locally (user IDs differ across sites).
• UI renders an Export button and an Import form on the Alerts list screen via the views_edit-wp_stream_alerts filter. Result counts show as admin notices.
• New filter wp_stream_alert_export_data for altering the export payload.

classes/class-admin.php

Instantiates Alert_Port in Admin::init() next to Export, with a matching public $alert_port property.

Why this approach

Alerts are a custom post type (wp_stream_alerts) with two post-meta rows (alert_type, alert_meta). Alerts::get_alerts() and Alert::save() already round-trip the full state, so portability is serialization plus a file form, not new DB code. Built as a standalone class (not on the Abilities API) so it works on every supported install, not only WP 6.9+ with the Abilities setting on. Import is append-only because alerts have no stable natural key (title is derived).

Testing

Test 1: Export

1. Create one or two alerts on Stream > Alerts.
2. Click Export above the list table.
3. Open the downloaded stream-alerts.json.
   Result: every alert is listed with its trigger and notifier settings, no ID/date/author.

Test 2: Import round-trip

1. Export the file.
2. Delete one alert on the Alerts screen.
3. Import the file.
   Result: the alert reappears with the original settings. Success notice shows imported/skipped counts.

Test 3: Unknown notifier type

1. Edit the JSON to set one alert alert_type to a value not registered on the install.
2. Import it.
   Result: that alert is skipped, others import. Notice reports the skipped count.

Test 4: Missing user on target site

1. Import a file where trigger_author is a user ID that does not exist.
   Result: the alert imports with author reset to "any user" (not skipped).

Automated: PHPCS (main and tests standards) clean. PHPUnit Test_Alert_Port passes, 7 tests / 29 assertions, covering export shape, import round-trip, unknown-type skip, missing-user reset, and non-array entry skip.