fix: ensure demo connection exists on startup (fixes Docker E2E)

Docker uses PostgreSQL with empty tables — no demo connection record existed.
Added ensure_demo_connection() that creates "Sample Database" when connections
table is empty. Called during startup alongside fix_demo_db_path().

Author: MoonMao42

apps/api/app/core/demo_db.py

@@ -124,6 +124,33 @@ def init_demo_database() -> str:
     return get_demo_db_path()
 
 
+async def ensure_demo_connection(db: AsyncSession, demo_db_path: str) -> None:
+    """Ensure a Sample Database connection exists. Creates one if the connections table is empty."""
+    from sqlalchemy import func
+
+    from app.services.app_settings import get_or_create_app_settings
+
+    count = await db.scalar(select(func.count(Connection.id)))
+    if count and count > 0:
+        return
+
+    conn = Connection(
+        name=DEMO_CONNECTION_NAME,
+        driver="sqlite",
+        database_name=demo_db_path,
+        extra_options={},
+        is_default=True,
+    )
+    db.add(conn)
+    await db.flush()
+
+    settings = await get_or_create_app_settings(db)
+    settings.default_connection_id = conn.id
+    settings.demo_initialized = True
+
+    logger.info("Created demo connection", name=DEMO_CONNECTION_NAME, path=demo_db_path)
+
+
 async def fix_demo_db_path(db: AsyncSession, demo_db_path: str) -> None:
     """修正预打包数据库中的占位符路径为实际的 demo.db 绝对路径"""
     result = await db.scalar(

apps/api/app/main.py

@@ -21,7 +21,7 @@
 
 from app.api.v1 import api_router
 from app.core.config import settings
-from app.core.demo_db import fix_demo_db_path, init_demo_database
+from app.core.demo_db import ensure_demo_connection, fix_demo_db_path, init_demo_database
 from app.db import AsyncSessionLocal, engine
 from app.db.base import Base
 from app.services.engine_diagnostics import categorize_sql_error
@@ -99,9 +99,10 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     # 确保 demo.db 存在（构建时已预生成，这里只是 fallback）
     demo_db_path = init_demo_database()
 
-    # 修正预打包数据库中的占位符路径
+    # 修正预打包数据库中的占位符路径，并确保 demo 连接存在
     async with AsyncSessionLocal() as session:
         await fix_demo_db_path(session, demo_db_path)
+        await ensure_demo_connection(session, demo_db_path)
         await session.commit()
 
     logger.info(