Add ActivityPub Move activity support for account migration

[Copilot]

Implements ActivityPub Move activity to migrate followers from GoBlog to another Fediverse account. Compatible servers (Mastodon, GoToSocial, etc.) automatically transfer follows to the new account.

CLI Commands

# Migrate followers (target must have GoBlog URL in alsoKnownAs)
./GoBlog activitypub move-followers <blog> <target>

# Clear movedTo setting to undo migration
./GoBlog activitypub clear-moved <blog>

Changes

ActivityPub types (pkgs/activitypub/)

• Add MoveType constant, Target field to Activity, MovedTo field to Person

Move implementation (activityPub.go, activityStreams.go)

• apMoveFollowers: validates target's alsoKnownAs, saves movedTo to DB, purges cache, sends Move to followers
• Actor profile includes movedTo when set

Database (settingsDb.go)

• Per-blog movedTo setting with getter/setter/delete

Tests

• Integration test with GoToSocial v0.20.3 verifying Move delivery and automatic follow transfer
• Unit tests for Move types and settings

Documentation (README.md)

• Migration instructions for moving to/from GoBlog

Notes

• Move activity addressed only to followers collection (not public) per GTS requirements
• GoBlog ignores incoming Move activities—it's publish-only and doesn't follow accounts

Original prompt

Please implement support for ActivityPub "Move" to move all followers of one account on GoBlog to another fediverse account (with a CLI command). Test that behaviour with integration tests (see the integration test with GoToSocial): There should be a second account on GTS and it should be confirmed that the first account moves the follow from GoBlog to the second account, when it receives the Move. Keep the code simple and understandable, but cover it with tests. Add documentation to the readme on how to move followers to an external account and how GoBlog can also be the target for an account move.

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