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/db/migrations/0003_activity_log.sql

@@ -0,0 +1,10 @@
+CREATE TABLE activity_log (
+    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+    host_id      INTEGER NOT NULL REFERENCES hosts(id) ON DELETE CASCADE,
+    operation_id INTEGER REFERENCES operations(id) ON DELETE SET NULL,
+    level        TEXT NOT NULL DEFAULT 'info',
+    message      TEXT NOT NULL,
+    source       TEXT NOT NULL DEFAULT '',
+    created_at   TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ','now'))
+);
+CREATE INDEX idx_activity_host ON activity_log(host_id, created_at DESC);