feat: self-service worker enrollment bound to a user account

Let a signed-in user turn their own machine into a worker without the
shared token. The coordinator already binds a JWT-authenticated
registration to owner_id as untrusted; this adds the missing pieces.

userservice: long-lived worker keys (scimesh_wk_live_*, hash-at-rest)
with create/list/revoke and a public /worker-tokens/exchange that trades
a key for a short-lived JWT carrying the owner current role/verified.

python worker: SCIMESH_WORKER_KEY + SCIMESH_USERSERVICE_URL; a token
provider exchanges the key and refreshes the JWT proactively and on 401,
so a long-running worker survives token expiry. Static bearer token path
is unchanged.

coordinator UI: an "add your machine" page that mints a key and shows a
ready-to-run command, proxying key management to the userservice; the
dashboard gains an owner-scoped "my machines" section.

docs: how to run a worker from your account, plus the untrusted/quorum/
verified trust model.

users/internal/transport/http/errors.go

@@ -48,6 +48,12 @@ func statusForError(err error) (int, string) {
 		return http.StatusUnauthorized, "invalid email or password"
 	case errors.Is(err, usecase.ErrUserNotFound):
 		return http.StatusNotFound, "user not found"
+	case errors.Is(err, usecase.ErrWorkerKeyNotFound):
+		return http.StatusNotFound, "worker key not found"
+	case errors.Is(err, usecase.ErrInvalidWorkerKey):
+		return http.StatusUnauthorized, "invalid worker key"
+	case errors.Is(err, domain.ErrWorkerKeyNameTooLong):
+		return http.StatusBadRequest, "worker key name is too long"
 	case errors.Is(err, usecase.ErrPasswordTooShort):
 		return http.StatusBadRequest, "password must be at least 8 characters"
 	case errors.Is(err, usecase.ErrPasswordTooLong):