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/fixtures/test_config.yaml

@@ -1,5 +1,6 @@
 # 测试配置
 database:
+  active: sql_server
   sql_server:
     host: localhost
     port: 1433
@@ -8,3 +9,9 @@ database:
     password: test_pass
     driver: "{ODBC Driver 18 for SQL Server}"
     trust_server_certificate: yes
+  postgresql:
+    host: localhost
+    port: 5432
+    database: TestDB
+    username: test_user
+    password: test_pass