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/store/activity.go

@@ -0,0 +1,50 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"time"
+
+	"provisioning/internal/model"
+)
+
+type Activity struct {
+	DB *sql.DB
+}
+
+func (s *Activity) Log(ctx context.Context, hostID, opID int64, level model.LogLevel, source, message string) (int64, error) {
+	res, err := s.DB.ExecContext(ctx, `
+		INSERT INTO activity_log(host_id, operation_id, level, message, source)
+		VALUES(?,?,?,?,?)
+	`, hostID, nullInt64(opID), level, message, source)
+	if err != nil {
+		return 0, fmt.Errorf("insert activity: %w", err)
+	}
+	return res.LastInsertId()
+}
+
+func (s *Activity) ListByHost(ctx context.Context, hostID int64, limit int) ([]model.ActivityEntry, error) {
+	if limit <= 0 {
+		limit = 50
+	}
+	rows, err := s.DB.QueryContext(ctx, `
+		SELECT id, host_id, COALESCE(operation_id, 0), level, message, source, created_at
+		FROM activity_log WHERE host_id = ? ORDER BY created_at DESC LIMIT ?
+	`, hostID, limit)
+	if err != nil {
+		return nil, fmt.Errorf("list activity: %w", err)
+	}
+	defer rows.Close()
+	var out []model.ActivityEntry
+	for rows.Next() {
+		var e model.ActivityEntry
+		var createdAt string
+		if err := rows.Scan(&e.ID, &e.HostID, &e.OperationID, &e.Level, &e.Message, &e.Source, &createdAt); err != nil {
+			return nil, fmt.Errorf("scan activity: %w", err)
+		}
+		e.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+		out = append(out, e)
+	}
+	return out, rows.Err()
+}