refactor: align backend layout and supabase infra

Consolidate backend modules/tests under the backend package while syncing Supabase compose/env config and related plans.

backend/src/core/http/response.py

@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+from http import HTTPStatus
+
+from pydantic import BaseModel
+
+
+class ProblemDetails(BaseModel):
+    type: str = "about:blank"
+    title: str
+    status: int
+    detail: str
+    instance: str | None = None
+
+
+def build_problem_details(
+    *,
+    status_code: int,
+    detail: str,
+    type_value: str = "about:blank",
+    title: str | None = None,
+    instance: str | None = None,
+) -> ProblemDetails:
+    resolved_title = title or HTTPStatus(status_code).phrase
+    return ProblemDetails(
+        type=type_value,
+        title=resolved_title,
+        status=status_code,
+        detail=detail,
+        instance=instance,
+    )