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>

app/models/finished_goods.py

@@ -20,7 +20,10 @@ class FinishedGoodsLocation(Base):
     zongpai_no: Mapped[str] = mapped_column(String(64), nullable=False)
     location_code: Mapped[str] = mapped_column(String(64), nullable=False)
     created_at: Mapped[datetime] = mapped_column(
-        DateTime, nullable=False, server_default=func.getdate()
+        DateTime,
+        nullable=False,
+        default=func.current_timestamp(),
+        server_default=func.current_timestamp(),
     )
 
 
@@ -36,7 +39,10 @@ class FinishedGoodsBox(Base):
     paichan_no: Mapped[str] = mapped_column(String(64), nullable=False)
     box_no: Mapped[int] = mapped_column(nullable=False)
     created_at: Mapped[datetime] = mapped_column(
-        DateTime, nullable=False, server_default=func.getdate()
+        DateTime,
+        nullable=False,
+        default=func.current_timestamp(),
+        server_default=func.current_timestamp(),
     )
 
 
@@ -53,5 +59,8 @@ class FinishedGoodsBoxItem(Base):
     zongpai_no: Mapped[str] = mapped_column(String(64), nullable=False)
     quantity: Mapped[float | None] = mapped_column(Numeric(18, 3), nullable=True)
     created_at: Mapped[datetime] = mapped_column(
-        DateTime, nullable=False, server_default=func.getdate()
+        DateTime,
+        nullable=False,
+        default=func.current_timestamp(),
+        server_default=func.current_timestamp(),
     )