feat: backend env config upgrade - multi-env (dev/prod), DB_* support, Docker compatible

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/config.py

@@ -1,13 +1,16 @@
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
+# 数据库 URL 由 config.settings 提供（环境变量优先，支持 DB_* 拼接，容器兼容）
+from config.settings import DATABASE_URL, DATABASE_URL_SYNC
+
 
 class Settings(BaseSettings):
     model_config = SettingsConfigDict(env_file=".env", env_file_encoding="utf-8", extra="ignore")
 
     api_host: str = "0.0.0.0"
     api_port: int = 8000
-    database_url: str = "postgresql+asyncpg://wecom:wecom_secret@localhost:5432/wecom_ai"
-    database_url_sync: str = "postgresql://wecom:wecom_secret@localhost:5432/wecom_ai"
+    database_url: str = DATABASE_URL
+    database_url_sync: str = DATABASE_URL_SYNC
 
     jwt_secret: str = "change-me"
     jwt_algorithm: str = "HS256"