fix: server_stats column names + remove hardcoded spamassassin unknown

system.php: INSERT used wrong column names (cpu_pct/ram_pct/disk_pct/
load_1m) — table has cpu_usage/ram_usage/disk_usage/load_avg matching
migration 007. Fatal PDOException was crashing stats API → all services
showing as unknown.

admin.js mailServer(): remove spamassassin row (not installed, was
permanently hardcoded to 'unknown').

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

panel/public/assets/js/admin.js

@@ -1506,7 +1506,7 @@ ${ips.length ? `
   <div class="card">
     <div class="card-header"><span class="card-title">Mail Services</span></div>
     <div style="padding:1.25rem">
-      ${[['postfix',mailStatus],['dovecot',doveStatus],['spamassassin','unknown']].map(([s,st]) => `
+      ${[['postfix',mailStatus],['dovecot',doveStatus]].map(([s,st]) => `
         <div style="display:flex;align-items:center;justify-content:space-between;padding:.6rem 0;border-bottom:1px solid var(--border)">
           <span>${s} ${Nova.badge(st,st==='active'?'green':'red')}</span>
           <div style="display:flex;gap:.5rem">