chore: 删除旧 Flask 项目文件，迁移到 FastAPI 架构

- 删除 gx_gp_monitor/ 旧项目目录（Flask CLI 架构）
- 删除 app.py（旧 WSGI 入口）
- 更新 alembic/env.py 为异步引擎（create_async_engine + run_sync）

alembic/env.py

@@ -1,34 +1,33 @@
+import asyncio
 from alembic import context
-from sqlalchemy import engine_from_config, pool
+from sqlalchemy.ext.asyncio import create_async_engine
 from app.models.announcement import Base
-
-config = context.config
+from app.config import settings
 
 target_metadata = Base.metadata
 
 
 def run_migrations_offline():
-    from app.config import settings
-    url = settings.database_url
-    context.configure(url=url, target_metadata=target_metadata, literal_binds=True)
+    context.configure(url=settings.database_url, target_metadata=target_metadata,
+                      literal_binds=True, dialect_opts={"paramstyle": "named"})
     with context.begin_transaction():
         context.run_migrations()
 
 
-def run_migrations_online():
-    from app.config import settings
-    connectable = engine_from_config(
-        {"sqlalchemy.url": settings.database_url},
-        prefix="sqlalchemy.",
-        poolclass=pool.NullPool,
-    )
-    with connectable.connect() as connection:
-        context.configure(connection=connection, target_metadata=target_metadata)
-        with context.begin_transaction():
-            context.run_migrations()
+def do_run_migrations(connection):
+    context.configure(connection=connection, target_metadata=target_metadata)
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_migrations_online():
+    connectable = create_async_engine(settings.database_url, echo=True)
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+    await connectable.dispose()
 
 
 if context.is_offline_mode():
     run_migrations_offline()
 else:
-    run_migrations_online()
+    asyncio.run(run_migrations_online())