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

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

backend/config/__init__.py

@@ -0,0 +1 @@
+# Config package: env-based settings, DATABASE_URL from env or DB_*.

backend/config/settings.py

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+"""
+统一配置加载：环境变量优先，fallback 到 .env，无 DATABASE_URL 时按 DB_* 拼接。
+支持多环境（dev/prod）、容器（DB_HOST=db）、向后兼容。
+"""
+import os
+from pathlib import Path
+
+# 环境变量优先；fallback 到 .env（不覆盖已存在的系统变量）
+def _load_dotenv():
+    try:
+        from dotenv import load_dotenv
+    except ImportError:
+        return
+    env = os.getenv("ENV", "").lower()
+    base = Path(__file__).resolve().parent.parent
+    if env == "prod":
+        load_dotenv(base / ".env.prod", override=False)
+    elif env == "dev":
+        load_dotenv(base / ".env.dev", override=False)
+    load_dotenv(base / ".env", override=False)
+
+
+_load_dotenv()
+
+
+def build_database_url_async() -> str:
+    """拼接异步数据库 URL（postgresql+asyncpg）。"""
+    user = os.getenv("DB_USER", "wecom")
+    password = os.getenv("DB_PASSWORD", "password")
+    host = os.getenv("DB_HOST", "localhost")
+    port = os.getenv("DB_PORT", "5432")
+    db = os.getenv("DB_NAME", "wecom_ai")
+    return f"postgresql+asyncpg://{user}:{password}@{host}:{port}/{db}"
+
+
+def build_database_url_sync() -> str:
+    """拼接同步数据库 URL（postgresql，Alembic/psycopg2）。"""
+    user = os.getenv("DB_USER", "wecom")
+    password = os.getenv("DB_PASSWORD", "password")
+    host = os.getenv("DB_HOST", "localhost")
+    port = os.getenv("DB_PORT", "5432")
+    db = os.getenv("DB_NAME", "wecom_ai")
+    return f"postgresql://{user}:{password}@{host}:{port}/{db}"
+
+
+# 优先使用环境变量，否则按 DB_* 拼接（向后兼容）
+DATABASE_URL: str = os.getenv("DATABASE_URL") or build_database_url_async()
+DATABASE_URL_SYNC: str = os.getenv("DATABASE_URL_SYNC") or build_database_url_sync()