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/v1/router.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+from fastapi import APIRouter
+
+from core.http.models import HealthResponse
+from v1.auth.router import router as auth_router
+from v1.infra.router import router as infra_router
+from v1.profile.router import router as profile_router
+
+
+router = APIRouter(prefix="/api/v1")
+router.include_router(auth_router)
+router.include_router(infra_router)
+router.include_router(profile_router)
+
+
+@router.get("/health", response_model=HealthResponse)
+async def health() -> HealthResponse:
+    return HealthResponse(status="ok")