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/tests/unit/test_response_envelope.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+from core.http.response import ProblemDetails, build_problem_details
+
+
+def test_problem_details_defaults() -> None:
+    result = build_problem_details(status_code=401, detail="Unauthorized")
+
+    assert isinstance(result, ProblemDetails)
+    assert result.type == "about:blank"
+    assert result.title == "Unauthorized"
+    assert result.status == 401
+    assert result.detail == "Unauthorized"
+    assert result.instance is None
+
+
+def test_problem_details_overrides() -> None:
+    result = build_problem_details(
+        status_code=409,
+        detail="Conflict",
+        type_value="https://example.com/problems/conflict",
+        title="Conflict",
+        instance="/api/mobile/auth/signup",
+    )
+
+    assert result.type == "https://example.com/problems/conflict"
+    assert result.title == "Conflict"
+    assert result.status == 409
+    assert result.detail == "Conflict"
+    assert result.instance == "/api/mobile/auth/signup"