refactor: switch password hashing from argon2 to bcrypt

- hashPassword now emits bcrypt (cost 12) instead of argon2id
- checkLogin migrates legacy md5/argon2id hashes to bcrypt on sign-in
- keep argon2id verification only as a one-time migration path
- replace ARGON2_* env vars with BCRYPT_COST

.env.example

@@ -32,9 +32,8 @@ 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
-ARGON2_MEMORY_KB=65536
-ARGON2_ITERATIONS=4
-ARGON2_PARALLELISM=1
+# CONVERT_PASSWORDS: enables legacy md5/argon2id -> bcrypt upgrade on login.
+BCRYPT_COST=12
 
 # --- PATHS ---
 BADGE_UPLOAD_DIR=./public/assets/images/badges