New external_identities table (UNIQUE(provider, external_id),
UNIQUE(provider, user_id)) + bind/initiate, bindings list/unbind
endpoints. Callback branches on bind_user_id in state: bind success
shows no key and no user id; conflicts are 409
(identity_bound_to_other / user_already_bound); repeat bind to the
same user+identity is idempotent. Login resolution checks bindings
first: a bound identity lands in the bound account and rotates its
key like any OAuth login; unbound identities keep the deterministic
oauth:<provider>:<login> mapping. Unbind restores oauth mapping.
Tests: new cloud/tests/test_oauth_bind.py (11). Full suites: host
29 OK, cloud 389 OK (4 pre-existing skips).
