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>

internal/server/server.go

@@ -53,6 +53,8 @@ type Server struct {
 	web     fs.FS
 	certs   *certReloader
 	statsQ  *stats.Querier
+	// oldSecretWarn limits warnings about rotated tokens' old secrets.
+	oldSecretWarn throttle
 
 	mu   sync.Mutex
 	srvs []*http.Server