Backend overhaul: new models, routers, schemas for shifts, business day, flags, messages, settings

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

local_backend/schemas/base.py

@@ -0,0 +1,14 @@
+from datetime import datetime
+from typing import Annotated
+from pydantic import PlainSerializer
+
+# SQLite strips tzinfo on read-back, so naive datetimes from DB are actually UTC.
+# This serializer appends "Z" so browsers parse them correctly as UTC.
+UTCDatetime = Annotated[
+    datetime,
+    PlainSerializer(
+        lambda dt: (dt.isoformat() + "Z") if dt.tzinfo is None else dt.isoformat(),
+        return_type=str,
+        when_used="json",
+    ),
+]