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/db/schema/index.js

@@ -10,6 +10,7 @@ import { sqliteTable, text, integer } from 'drizzle-orm/sqlite-core';
  * @property {string|null} lotwPassword
  * @property {string|null} dclApiKey
  * @property {boolean} isAdmin
+ * @property {Date|null} lastSeen
  * @property {Date} createdAt
  * @property {Date} updatedAt
  */
@@ -23,6 +24,7 @@ export const users = sqliteTable('users', {
   lotwPassword: text('lotw_password'), // Encrypted
   dclApiKey: text('dcl_api_key'), // DCL API key for future use
   isAdmin: integer('is_admin', { mode: 'boolean' }).notNull().default(false),
+  lastSeen: integer('last_seen', { mode: 'timestamp' }),
   createdAt: integer('created_at', { mode: 'timestamp' }).notNull().$defaultFn(() => new Date()),
   updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull().$defaultFn(() => new Date()),
 });