fix: 修复3个问题

1. 对比度: --text-muted 从 #94a3b8 改为 #c8d6e5 (文字更亮)
2. 登录: auth.py 从 .env 文件读取密码，解决后台进程环境变量丢失
3. 实例创建: 同步 instances.py 的 db import (之前缺 import db)

auth.py

@@ -7,10 +7,23 @@ bp = Blueprint("auth", __name__)
 
 
 def _get_admin_creds():
-    return (
-        os.environ.get("SM_ADMIN_USER", "admin"),
-        os.environ.get("SM_ADMIN_PASSWORD", ""),
-    )
+    import os
+    _ENV = "/root/socks-manager/.env"
+    pwd = os.environ.get("SM_ADMIN_PASSWORD")
+    user = os.environ.get("SM_ADMIN_USER", "admin")
+    if not pwd and os.path.exists(_ENV):
+        try:
+            for line in open(_ENV):
+                line = line.strip()
+                if line and not line.startswith("#"):
+                    k, _, v = line.partition("=")
+                    if k.strip() == "SM_ADMIN_PASSWORD":
+                        pwd = v.strip()
+                    elif k.strip() == "SM_ADMIN_USER":
+                        user = v.strip()
+        except Exception:
+            pass
+    return (user, pwd or "")
 
 
 def login_required(f):