feat: add PostgreSQL database backend support

Add multi-database support allowing selection between SQL Server and
PostgreSQL via database.active config. Changes include dialect-aware
SQL generation, cross-database timestamp functions, PostgreSQL connection
URL builder, and psycopg dependency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_config.py

@@ -5,6 +5,7 @@ from config.settings import load_settings
 def test_load_settings_success():
     """测试成功加载配置文件"""
     settings = load_settings("tests/fixtures/test_config.yaml")
+    assert settings.database.active == "sql_server"
     assert settings.database.sql_server.port == 1433
     assert settings.database.sql_server.host == "localhost"
     assert settings.database.sql_server.database == "TestDB"
@@ -23,3 +24,14 @@ def test_database_url_property():
     assert "mssql+pyodbc" in str(url)
     assert "test_user" in str(url)
     assert "TestDB" in str(url)
+
+
+def test_postgresql_database_url_property():
+    """测试 PostgreSQL database_url 属性生成"""
+    settings = load_settings("tests/fixtures/test_config.yaml")
+    settings.database.active = "postgresql"
+    url = settings.database_url
+    assert url.drivername == "postgresql+psycopg"
+    assert url.host == "localhost"
+    assert url.port == 5432
+    assert url.database == "TestDB"