Update admin theme/layout and refresh README details.

Align the project baseline with the latest admin interface styling and layout structure while documenting setup and usage updates in README.

apps/api/app/core/datetime_utils.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+
+def ensure_utc(value: datetime | None) -> datetime | None:
+    if value is None:
+        return None
+    if value.tzinfo is None:
+        return value.replace(tzinfo=UTC)
+    return value.astimezone(UTC)
+
+
+def utc_now() -> datetime:
+    return datetime.now(UTC)