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/setup_wizard.py

@@ -76,6 +76,33 @@ def main() -> int:
 
         execution = run_execution_step(f"Step 3/{total_steps}")
 
+        # Database backend (Stage 0+ recommends Postgres; SQLite kept as
+        # offline dev fallback). Inline question rather than a separate
+        # wizard step — minimal addition, full step module can come later.
+        print()
+        print_header("Database backend")
+        print("Stage 0+ recommends Postgres for parity with production.")
+        print("SQLite is kept as an offline dev fallback.")
+        print()
+        use_postgres = ask_yes_no("Use Postgres? (y = postgres, n = sqlite)", default=True)
+        database_backend = "postgres" if use_postgres else "sqlite"
+        database_url: str | None = None
+        if use_postgres:
+            print()
+            print_info(
+                "Set DATABASE_URL in your .env file. Example:\n"
+                "  postgresql+asyncpg://deerflow:deerflow_dev@localhost:5432/deerflow\n"
+                "Or for a remote RDS:\n"
+                "  postgresql+asyncpg://USER:PASS@HOST:5432/DB"
+            )
+            print()
+            try:
+                raw = input("DATABASE_URL (leave blank to set later in .env): ").strip()
+            except EOFError:
+                raw = ""
+            if raw:
+                database_url = raw
+
         print_header(f"Step {total_steps}/{total_steps} · Writing configuration")
 
         write_config_yaml(
@@ -97,6 +124,7 @@ def main() -> int:
             allow_host_bash=execution.allow_host_bash,
             include_bash_tool=execution.include_bash_tool,
             include_write_tools=execution.include_write_tools,
+            database_backend=database_backend,
         )
         print_success(f"Config written to: {config_path.relative_to(project_root)}")
 
@@ -113,6 +141,8 @@ def main() -> int:
             env_pairs[search_provider.env_var] = search_api_key
         if fetch_api_key and fetch_provider and fetch_provider.env_var:
             env_pairs[fetch_provider.env_var] = fetch_api_key
+        if database_url:
+            env_pairs["DATABASE_URL"] = database_url
 
         if env_pairs:
             write_env_file(env_path, env_pairs)

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")