feat(auth): implement migration script for auth secret and refactor secret handling

- Added a new script `migrate-auth-secret.ts` to facilitate the migration of 2FA secrets when changing the BETTER_AUTH_SECRET.
- Updated `package.json` to include a command for running the migration script.
- Refactored the handling of BETTER_AUTH_SECRET to improve security by removing the hardcoded default and introducing a fallback mechanism using environment variables or Docker secrets.
- Updated the authentication logic to utilize the new `betterAuthSecret` function for retrieving the secret.

packages/server/src/db/constants.ts

@@ -9,7 +9,7 @@ export const {
 	POSTGRES_PORT = "5432",
 } = process.env;
 
-function readSecret(path: string): string {
+export function readSecret(path: string): string {
 	try {
 		return fs.readFileSync(path, "utf8").trim();
 	} catch {