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.py

@@ -1,7 +1,7 @@
 from pathlib import Path
-from typing import Optional
+from typing import Literal
 from sqlalchemy.engine import URL
-from pydantic import BaseModel, Field
+from pydantic import BaseModel
 from pydantic_yaml import parse_yaml_raw_as
 
 
@@ -16,9 +16,20 @@ class SqlServerConfig(BaseModel):
     trust_server_certificate: str = "yes"
 
 
+class PostgreSqlConfig(BaseModel):
+    """PostgreSQL 连接配置"""
+    host: str
+    port: int = 5432
+    database: str
+    username: str
+    password: str
+
+
 class DatabaseConfig(BaseModel):
     """数据库配置"""
+    active: Literal["sql_server", "postgresql"] = "sql_server"
     sql_server: SqlServerConfig
+    postgresql: PostgreSqlConfig | None = None
 
 
 class Settings(BaseModel):
@@ -28,6 +39,11 @@ class Settings(BaseModel):
     @property
     def database_url(self) -> URL:
         """构建数据库连接 URL"""
+        if self.database.active == "postgresql":
+            return self._postgresql_url()
+        return self._sql_server_url()
+
+    def _sql_server_url(self) -> URL:
         conf = self.database.sql_server
         return URL.create(
             "mssql+pyodbc",
@@ -42,6 +58,19 @@ class Settings(BaseModel):
             },
         )
 
+    def _postgresql_url(self) -> URL:
+        conf = self.database.postgresql
+        if conf is None:
+            raise ValueError("已选择 postgresql，但未配置 database.postgresql")
+        return URL.create(
+            "postgresql+psycopg",
+            username=conf.username,
+            password=conf.password,
+            host=conf.host,
+            port=conf.port,
+            database=conf.database,
+        )
+
 
 def load_settings(config_path: str = "config/settings.yaml") -> Settings:
     """加载 YAML 配置文件"""