feat(auth): add admin bootstrap and username login

Initialize admin bootstrap settings during startup, persist username support in auth flows, and align frontend auth requests with local API behavior.

backend/.env.example

@@ -16,10 +16,18 @@ DATA_DIR=./data
 CHROMA_PERSIST_DIR=./data/chroma
 UPLOAD_DIR=./data/uploads
 MAX_UPLOAD_SIZE=52428800
+# Supported values: ch | en
+MINERU_LANGUAGE=ch
 
 # === JWT ===
 ACCESS_TOKEN_EXPIRE_MINUTES=1440
 
+# === 管理员账号 Bootstrap ===
+ADMIN=admin
+ADMIN_EMAIL=admin@example.com
+ADMIN_PASSWORD=
+ADMIN_FULL_NAME=Administrator
+
 # === 定时任务 ===
 SCHEDULER_ENABLED=true
 DAILY_PLAN_TIME=00:00