feat: guest passage — slip through as a wanderer

POST /auth/guest mints a real user row (wanderer-<4 hex>, collision
retry, unusable random password) and issues the normal session cookie,
per-IP rate limited at 5/hour. EnterPage gains the guest action;
the séance shows a dismissible claim-a-name note for wanderer- users.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/SeancePage.css

@@ -329,6 +329,50 @@
 
 /* ---------- connection banner ---------- */
 
+.seance-wanderer-note {
+  position: absolute;
+  top: 62px;
+  left: 50%;
+  transform: translateX(-50%);
+  z-index: 29;
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.4rem 0.9rem;
+  background: rgba(16, 16, 26, 0.92);
+  border: 1px dashed rgba(216, 230, 221, 0.3);
+  border-radius: 3px;
+  color: rgba(216, 230, 221, 0.65);
+  font-size: 0.72rem;
+  letter-spacing: 0.06em;
+  white-space: nowrap;
+}
+
+.seance-wanderer-note a {
+  color: #8dffc0;
+  text-decoration: none;
+  border-bottom: 1px dotted rgba(141, 255, 192, 0.5);
+}
+
+.seance-wanderer-note a:hover {
+  border-bottom-style: solid;
+}
+
+.wanderer-note-dismiss {
+  margin-left: 0.2rem;
+  background: transparent;
+  border: none;
+  color: rgba(216, 230, 221, 0.45);
+  font-size: 0.9rem;
+  line-height: 1;
+  cursor: pointer;
+  padding: 0 0.2rem;
+}
+
+.wanderer-note-dismiss:hover {
+  color: #d8e6dd;
+}
+
 .seance-conn-banner {
   position: absolute;
   top: 62px;