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>

package.json

@@ -8,7 +8,6 @@
     "start": "node src/index.js",
     "test": "node --experimental-vm-modules node_modules/.bin/jest",
     "db:generate": "drizzle-kit generate",
-    "db:migrate": "node src/db/migrate.js",
     "db:push": "drizzle-kit push",
     "db:studio": "drizzle-kit studio",
     "db:seed": "node src/db/seeds/admin.seed.js",
@@ -20,7 +19,6 @@
   "dependencies": {
     "axios": "^1.13.2",
     "bcryptjs": "^3.0.3",
-    "better-auth": "^1.3.34",
     "cookie-parser": "^1.4.7",
     "cors": "^2.8.5",
     "dotenv": "^16.4.5",