fix(security): eliminate default admin password vulnerability

BREAKING CHANGE: Admin password is now auto-generated on first setup

Security improvements:
- Remove hardcoded 'admin123' password completely
- Generate secure random password on first installation
- Save credentials to ADMIN_CREDENTIALS.txt (git-ignored)
- Force password change on first login
- Implement strong password requirements (12+ chars, mixed case, numbers, special)
- Add password strength validation
- Increase bcrypt rounds from 10 to 12

New features:
- Password generator utility with secure random generation
- Human-readable password format (e.g., SwiftEagle3847\!)
- Password reset script for existing installations
- Comprehensive admin setup documentation
- Must-change-password flag in database

Migration guide:
- New installations: Check ADMIN_CREDENTIALS.txt for generated password
- Existing installations: Run scripts/reset-admin-password.js
- All users must change password on first login after update

This fixes a critical vulnerability where all installations used the same
default admin password, allowing unauthorized access.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/src/routes/auth.js

@@ -41,14 +41,15 @@ router.post('/admin/login', [
     // Update last login
     await db('admin_users').where('id', admin.id).update({ last_login: new Date() });
     
-    const token = jwt.sign({ id: admin.id }, process.env.JWT_SECRET, { expiresIn: '24h' });
+    const token = jwt.sign({ id: admin.id, type: 'admin' }, process.env.JWT_SECRET, { expiresIn: '24h' });
     
     res.json({
       token,
       user: {
         id: admin.id,
         username: admin.username,
-        email: admin.email
+        email: admin.email,
+        mustChangePassword: admin.must_change_password || false
       }
     });
   } catch (error) {