feat(wizard): add Postgres backend question to setup wizard

setup_wizard.py 在 Step 3 (Execution) 之后新增一个 Database 问答：
"Use Postgres? (y = postgres, n = sqlite)"，默认 y（Stage 0+ 推荐 PG）。
选 y 时引导填 DATABASE_URL（可留空稍后写 .env）；DATABASE_URL 进 .env，
config.yaml 写入 database.backend=postgres + postgres_url=\$DATABASE_URL。

writer.py build_minimal_config 加 database_backend 参数，postgres 时
覆盖 base_config 的 database 段；默认 sqlite 时沿用 base_config 行为
(继承 config.example.yaml 的 sqlite_dir 等)。

minimal pattern：不新建 wizard/steps/database.py，inline 在 main 里加 1
问答 + writer 加 1 参数。后续如果需要更复杂数据库选项再升为完整 step 模块。

Stage 0 PR1 T1.8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/wizard/writer.py

@@ -172,8 +172,14 @@ def build_minimal_config(
     include_write_tools: bool = True,
     config_version: int = 5,
     base_config: dict[str, Any] | None = None,
+    database_backend: str = "sqlite",
 ) -> str:
-    """Build the content of a minimal config.yaml."""
+    """Build the content of a minimal config.yaml.
+
+    ``database_backend``: ``"sqlite"`` (default; reuses base_config's sqlite_dir)
+    or ``"postgres"`` (writes ``backend: postgres`` + ``postgres_url: $DATABASE_URL``;
+    ``DATABASE_URL`` must be set in ``.env``).
+    """
     from datetime import date
 
     today = date.today().isoformat()
@@ -220,6 +226,13 @@ def build_minimal_config(
         sandbox_config.pop("allow_host_bash", None)
     data["sandbox"] = sandbox_config
 
+    # Override database backend if user chose postgres in the wizard.
+    if database_backend == "postgres":
+        data["database"] = {
+            "backend": "postgres",
+            "postgres_url": "$DATABASE_URL",
+        }
+
     header = (
         f"# DeerFlow Configuration\n"
         f"# Generated by 'make setup' on {today}\n"
@@ -250,6 +263,7 @@ def write_config_yaml(
     allow_host_bash: bool = False,
     include_bash_tool: bool = False,
     include_write_tools: bool = True,
+    database_backend: str = "sqlite",
 ) -> None:
     """Write (or overwrite) config.yaml with a minimal working configuration."""
     # Read config_version from config.example.yaml if present
@@ -286,5 +300,6 @@ def write_config_yaml(
         include_write_tools=include_write_tools,
         config_version=config_version,
         base_config=example_defaults,
+        database_backend=database_backend,
     )
     config_path.write_text(content, encoding="utf-8")