feat: admin spam

[shaunwarman]

• Summary cards — total spam count (last 24h, 7d, 30d), top offending IPs, top error messages
• Spam over time chart — ApexCharts area chart showing spam volume bucketed by hour/day, filterable by IP and date range
• Filterable log table — standard paginated table (like /admin/logs) pre-filtered to bounce_category: 'spam', with additional filter controls for IP, hostname, date range, and error message search
• Top bad actors section — aggregation showing IPs/domains/senders with highest spam bounce counts