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>

config/settings.yaml.example

@@ -1,5 +1,6 @@
 # 数据库配置
 database:
+  active: sql_server  # 可选: sql_server, postgresql
   sql_server:
     host: YOUR_DB_HOST
     port: 1433
@@ -8,3 +9,9 @@ database:
     password: YOUR_PASSWORD
     driver: "{ODBC Driver 18 for SQL Server}"
     trust_server_certificate: yes
+  postgresql:
+    host: YOUR_POSTGRES_HOST
+    port: 5432
+    database: YOUR_POSTGRES_DB_NAME
+    username: YOUR_POSTGRES_USERNAME
+    password: YOUR_POSTGRES_PASSWORD