feat(auth): switch password hashing to argon2id with legacy auto-upgrade

- hashPassword now emits argon2id (same params as the legacy AtomCMS
  Laravel setup: memory 64MB, iterations 4, parallelism 1)
- legacy md5 and bcrypt hashes are verified and auto-upgraded to
  argon2id on successful login (CONVERT_PASSWORDS=true)
- replace BCRYPT_ROUNDS env with ARGON2_MEMORY_KB / ARGON2_ITERATIONS /
  ARGON2_PARALLELISM
- update README and add tests for argon2id and bcrypt upgrade paths

src/lib/auth.ts

@@ -176,7 +176,7 @@ export const { handlers, signOut, auth } = NextAuth({
 					return null;
 				}
 
-				// Byte-compatible AtomCMS check (bcrypt + md5->bcrypt upgrade).
+				// Byte-compatible AtomCMS check (argon2id + legacy md5/bcrypt upgrade).
 				if (!user.password) return null;
 				const res = await checkLogin(password, user.password, {
 					convertPasswords: env.CONVERT_PASSWORDS,