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

.env.example

@@ -32,7 +32,9 @@ NEXT_PUBLIC_IMAGER_URL=http://localhost:3002/imaging
 AUTH_SECRET=your-super-secret-auth-key-change-this-min-32-chars
 APP_KEY=base64:your-app-key-here=
 CONVERT_PASSWORDS=true
-BCRYPT_ROUNDS=12
+ARGON2_MEMORY_KB=65536
+ARGON2_ITERATIONS=4
+ARGON2_PARALLELISM=1
 
 # --- PATHS ---
 BADGE_UPLOAD_DIR=./public/assets/images/badges