Add Arrive outbox handler for check-in activities

[pfefferle]

Fixes #

Proposed changes:

• Add an Arrive outbox handler that creates WordPress posts from C2S check-in activities (e.g. from checkin.swf.pub).
• The handler synthesizes a Note from the intransitive Arrive activity, creates a post via Posts::create(), and saves location geodata (geo_address, geo_latitude, geo_longitude, geo_public) via the activitypub_outbox_arrive_sent action hook.
• Fix the outbox controller to fall back to Create when looking up outbox entries, since the Post scheduler wraps new posts in Create activities regardless of the original C2S activity type (e.g. Arrive → Create).

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Start wp-env with npm run env-start
• Run npm run env-test -- --filter=test_c2s_arrive — both tests should pass
• Alternatively, use a C2S client like checkin.swf.pub to POST an Arrive activity to the outbox endpoint and verify a WordPress post is created with geodata meta

Changelog entry

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Changelog Entry Details

Significance

• Patch

Type

• Added - for new features

Message

Add support for check-in activities posted via compatible apps.