Add activity log system for provisioning lifecycle visibility

Hosts stuck in states like pxe_ready had zero visibility into why.
This adds a persistent activity log that records every meaningful
step (state transitions, PXE events, cluster join stages, failures)
and surfaces it on the host detail page with live SSE updates.
Includes a stuck-detection warning banner when a host sits in
pxe_ready for >10 minutes with no iPXE request.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/static/app.css

@@ -391,6 +391,48 @@ main {
 .progress-text { font-size: 0.825rem; color: var(--text); font-weight: 500; }
 .progress-detail { font-size: 0.775rem; color: var(--text-secondary); }
 
+/* === STUCK WARNING === */
+.stuck-warning {
+  background: var(--amber-bg);
+  color: var(--amber);
+  border: 1px solid var(--amber-border);
+  padding: 0.75rem 1rem;
+  border-radius: var(--radius-sm);
+  margin-bottom: 1rem;
+  font-size: 0.825rem;
+  font-weight: 500;
+  line-height: 1.5;
+}
+
+/* === ACTIVITY LOG === */
+.activity-log { max-height: 400px; overflow-y: auto; }
+.log-entry {
+  display: flex;
+  align-items: baseline;
+  gap: 0.75rem;
+  padding: 0.4rem 0.75rem;
+  border-bottom: 1px solid var(--border);
+  font-size: 0.825rem;
+}
+.log-entry:last-child { border-bottom: none; }
+.log-time {
+  color: var(--text-tertiary);
+  font-family: var(--font-mono);
+  font-size: 0.75rem;
+  min-width: 3.5rem;
+  flex-shrink: 0;
+}
+.log-source {
+  color: var(--text-tertiary);
+  font-family: var(--font-mono);
+  font-size: 0.75rem;
+  min-width: 5rem;
+  flex-shrink: 0;
+}
+.log-msg { color: var(--text); }
+.log-warn .log-msg { color: var(--amber); }
+.log-error .log-msg { color: var(--red); font-weight: 500; }
+
 /* === UTILITY === */
 .inline { display: inline; }