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/conftest.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+
+def pytest_configure() -> None:
+    root = Path(__file__).resolve().parents[2]
+    src_path = root / "backend" / "src"
+    if str(src_path) not in sys.path:
+        sys.path.insert(0, str(src_path))