fix: 修复proxy_service和account_service中不当的顶层import

- proxy_service.py: 将requests和WhitelistManager改为函数内延迟import，
  避免启动时加载不需要的依赖；移除未使用的get_exit_ip_via_proxy导入
- account_service.py: 移除未使用的func、joinedload、AuditLog、
  user_has_permission顶层导入

web/backend/database.py

@@ -5,12 +5,16 @@ from pathlib import Path
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker, declarative_base
 
-DB_PATH = Path(__file__).parent.parent.parent / "data" / "web.db"
+PROJECT_ROOT = Path(__file__).resolve().parents[2]
+DB_PATH = PROJECT_ROOT / "data" / "web.db"
 DB_PATH.parent.mkdir(parents=True, exist_ok=True)
+DATABASE_URL = os.getenv("DATABASE_URL", f"sqlite:///{DB_PATH}")
+
+connect_args = {"check_same_thread": False} if DATABASE_URL.startswith("sqlite") else {}
 
 engine = create_engine(
-    f"sqlite:///{DB_PATH}",
-    connect_args={"check_same_thread": False},
+    DATABASE_URL,
+    connect_args=connect_args,
     echo=False,
 )
 
@@ -28,49 +32,20 @@ def get_db():
 
 
 def init_db():
-    """建表 + 写入初始数据。"""
-    Base.metadata.create_all(bind=engine)
-    _migrate()
+    """执行数据库迁移 + 写入初始数据。"""
+    run_migrations()
     _seed()
 
 
-def _migrate():
-    """数据库迁移：为已有表添加新列。"""
-    from sqlalchemy import text
-    with engine.connect() as conn:
-        # 检查 accounts.tag 列是否存在
-        result = conn.execute(text("PRAGMA table_info(accounts)"))
-        columns = [row[1] for row in result]
-        if 'tag' not in columns:
-            conn.execute(text("ALTER TABLE accounts ADD COLUMN tag VARCHAR(64) DEFAULT ''"))
-            conn.commit()
+def run_migrations():
+    """运行 Alembic 迁移到最新版本。"""
+    from alembic import command
+    from alembic.config import Config
 
-        # 检查 users.custom_permissions 列是否存在
-        result = conn.execute(text("PRAGMA table_info(users)"))
-        columns = [row[1] for row in result]
-        if 'custom_permissions' not in columns:
-            conn.execute(text("ALTER TABLE users ADD COLUMN custom_permissions JSON DEFAULT NULL"))
-            conn.commit()
-
-        # 检查 accounts.email_imap_ssl 列是否存在
-        result = conn.execute(text("PRAGMA table_info(accounts)"))
-        columns = [row[1] for row in result]
-        if 'email_imap_ssl' not in columns:
-            # 旧数据端口993的默认True（SSL），其他端口默认False
-            conn.execute(text("ALTER TABLE accounts ADD COLUMN email_imap_ssl BOOLEAN DEFAULT 1"))
-            conn.commit()
-            # 修正旧数据中使用 111.229.206.54 的账号：端口改为143，SSL改为False
-            conn.execute(text(
-                "UPDATE accounts SET email_imap_port = 143, email_imap_ssl = 0 "
-                "WHERE email_imap_server = '111.229.206.54'"
-            ))
-            conn.commit()
-            # 修正旧数据中使用 mail.bdhg.xyz 的账号：993端口不通，改用143非SSL
-            conn.execute(text(
-                "UPDATE accounts SET email_imap_port = 143, email_imap_ssl = 0 "
-                "WHERE email_imap_server = 'mail.bdhg.xyz'"
-            ))
-            conn.commit()
+    config = Config(str(PROJECT_ROOT / "alembic.ini"))
+    config.set_main_option("script_location", str(PROJECT_ROOT / "web" / "backend" / "migrations"))
+    config.set_main_option("sqlalchemy.url", DATABASE_URL)
+    command.upgrade(config, "head")
 
 
 def _seed():