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/migrations/add-last-seen.js

@@ -0,0 +1,86 @@
+/**
+ * Migration: Add last_seen column to users table
+ *
+ * This script adds the last_seen column to track when users last accessed the tool.
+ */
+
+import Database from 'bun:sqlite';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
+
+// ES module equivalent of __dirname
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+
+const dbPath = join(__dirname, '../award.db');
+const sqlite = new Database(dbPath);
+
+async function migrate() {
+  console.log('Starting migration: Add last_seen column...');
+
+  try {
+    // Check if last_seen column already exists
+    const columnExists = sqlite.query(`
+      SELECT COUNT(*) as count
+      FROM pragma_table_info('users')
+      WHERE name='last_seen'
+    `).get();
+
+    if (columnExists.count > 0) {
+      console.log('Column last_seen already exists. Skipping...');
+    } else {
+      // Add last_seen column
+      sqlite.exec(`
+        ALTER TABLE users
+        ADD COLUMN last_seen INTEGER
+      `);
+
+      console.log('Added last_seen column to users table');
+    }
+
+    console.log('Migration complete! last_seen column added to database.');
+  } catch (error) {
+    console.error('Migration failed:', error);
+    sqlite.close();
+    process.exit(1);
+  }
+
+  sqlite.close();
+}
+
+async function rollback() {
+  console.log('Starting rollback: Remove last_seen column...');
+
+  try {
+    // SQLite doesn't support DROP COLUMN directly before version 3.35.5
+    // For older versions, we need to recreate the table
+    console.log('Note: SQLite does not support DROP COLUMN. Manual cleanup required.');
+    console.log('To rollback: Recreate users table without last_seen column');
+
+    // For SQLite 3.35.5+, you can use:
+    // sqlite.exec(`ALTER TABLE users DROP COLUMN last_seen`);
+
+    console.log('Rollback note issued.');
+  } catch (error) {
+    console.error('Rollback failed:', error);
+    sqlite.close();
+    process.exit(1);
+  }
+
+  sqlite.close();
+}
+
+// Check if this is a rollback
+const args = process.argv.slice(2);
+if (args.includes('--rollback') || args.includes('-r')) {
+  rollback().then(() => {
+    console.log('Rollback script completed');
+    process.exit(0);
+  });
+} else {
+  // Run migration
+  migrate().then(() => {
+    console.log('Migration script completed successfully');
+    process.exit(0);
+  });
+}