Add the in-game /felis migrate command that a player runs to open an
account migration, the entry point for handing their owned servers to
another account (§B3 inherit, scenario A). The command posts the player's
Mojang-verified UUID to the existing handleMigrateStart backend, which puts
the account into migrate mode; the player then finishes on the web console
(prove identity, name the receiving account, redeem a one-time code).
Mirrors the existing /felis claim path: requires a real player past login
limbo, acts on the caller's account rather than the current server, expects
201 Created affirming started=true (a 201 without it is a contract breach,
not a refusal), and maps the handler refusals (404 not_linked, 409
account_retired) to player-facing guidance. On success it points the player
at https://console.<root_domain>, derived from config, never hardcoded.
Compile-verified against velocity-api:3.3.0-SNAPSHOT via the podman gradle
toolchain. Closes the code-only gap named in handlers_account_migrate.go.
flyemoji
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