fix(migration): prevent duplicate actor fields

[faisalahammad]

Fixes #3339

Proposed changes:

• Prevent the "Powered by WordPress" extra field from being added more than once per actor. The migration seeder now skips insertion when a field with the same title already exists.
• Add a migration step that runs once on upgrade and removes duplicate "Powered by" fields for the blog actor and every ActivityPub-capable user, keeping the oldest one.
• The cleanup runs only when moving to the unreleased version, so fresh installs are unaffected and existing duplicates collapse to a single field.

Other information:

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

Testing instructions:

• Install the plugin on a site that already has several duplicate "Powered by" fields in Settings → ActivityPub.
• Run the upgrade (activate the updated plugin).
• Confirm the field appears only once for the blog actor and for each user.
• On a fresh install, confirm exactly one "Powered by" field is created per actor.
• Delete the field manually, then run migrations again and confirm it is not re-added.

Changelog entry

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Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message

Fixed the "Powered by WordPress" profile field being added multiple times, and cleaned up any existing duplicates on update.