Security improvements, role in user creation, todo filters fix

- Remove better-auth dependency (unused)
- Update JWT secrets to stronger values
- Add ENCRYPTION_SALT env variable for password encryption
- Add role field to createUserSchema validator
- Accept role from body in admin.controller createUser
- Fix todo filters: add priority filter, handle completed param
- Remove .env.example (merged into .env)

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

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

src/validators/auth.validators.js

@@ -75,6 +75,7 @@ export const createUserSchema = z.object({
   emailPassword: z.string().min(1).optional(),
   firstName: z.string().max(100).optional(),
   lastName: z.string().max(100).optional(),
+  role: z.enum(['admin', 'member']).optional(),
 });
 
 // Update user schema