feat: add last_seen tracking for users

Adds last_seen field to track when users last accessed the tool:
- Add lastSeen column to users table schema (nullable timestamp)
- Create migration to add last_seen column to existing databases
- Add updateLastSeen() function to auth.service.js
- Update auth derive middleware to update last_seen on each authenticated request (async, non-blocking)
- Add lastSeen to admin getUserStats() query for display in admin users table
- Add "Last Seen" column to admin users table in frontend

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

src/backend/services/admin.service.js

@@ -127,6 +127,7 @@ export async function getUserStats() {
       email: users.email,
       callsign: users.callsign,
       isAdmin: users.isAdmin,
+      lastSeen: users.lastSeen,
       qsoCount: sql`CAST(COUNT(${qsos.id}) AS INTEGER)`,
       lotwConfirmed: sql`CAST(SUM(CASE WHEN ${qsos.lotwQslRstatus} = 'Y' THEN 1 ELSE 0 END) AS INTEGER)`,
       dclConfirmed: sql`CAST(SUM(CASE WHEN ${qsos.dclQslRstatus} = 'Y' THEN 1 ELSE 0 END) AS INTEGER)`,
@@ -144,10 +145,13 @@ export async function getUserStats() {
     .groupBy(users.id)
     .orderBy(sql`COUNT(${qsos.id}) DESC`);
 
-  // Convert lastSync timestamps (seconds) to Date objects for JSON serialization
+  // Convert timestamps (seconds) to Date objects for JSON serialization
+  // Note: lastSeen from Drizzle is already a Date object (timestamp mode)
+  // lastSync is raw SQL returning seconds, needs conversion
   return stats.map(stat => ({
     ...stat,
     lastSync: stat.lastSync ? new Date(stat.lastSync * 1000) : null,
+    // lastSeen is already a Date object from Drizzle, don't convert
   }));
 }