Add token rotation

jarl token rotate, a Rotate button in the web UI and
POST /api/v1/tokens/{id}/rotate give a token a new secret while it keeps
its ID, name, user and grants. The old secret stops at once or after a
grace period of up to 7 days, during which jarld logs who still uses it.
A token that expires gets its lifetime again. --save writes the new
secret where the CLI got the old one (its configuration or --token @FILE),
--write FILE to any file, and --current picks the token the CLI runs
with, so a restricted token can renew itself. A restricted token can't
rotate a wider token of its user.

Also fixes token create --save for named servers, and future times
(token expiry) shown as "just now" in the web UI.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/api.md

@@ -315,6 +315,7 @@ Success is `200 {"ok": true, "fqdn": ..., "changeset_id": ..., "zones": [...]}`.
 | GET / POST | `/me/tokens` | `{"name": "laptop", "role": "", "grant_strings": [...], "expires_in": "90d"}`. The response's `token` field is the secret, shown once |
 | GET | `/tokens` | every token (admin); `?all=1` includes revoked tokens |
 | DELETE | `/tokens/{id}` | revoke |
+| POST | `/tokens/{id}/rotate` | New secret, same token. Optional body `{"grace": "1h", "expires_in": "90d", "expires_at": "...", "no_expiry": false}`. The response's `token` field is the new secret, shown once; `previous_secret_valid_until` says when the old one stops. A restricted token may rotate itself and tokens no wider than itself |
 | POST | `/me/password` | `{"current_password": "...", "password": "..."}` (sessions only) |
 
 User and token changes take effect immediately. They are not changesets,