Stage 2: Add FastAPI shell with Flask fallback

[anth-volk]

Goal

Implement Stage 2 of the API v2-alpha migration plan: add a FastAPI ASGI shell that can host the existing Flask API through WSGI fallback without changing the public API contract.

Targets

• Add a FastAPI ASGI entrypoint.
• Mount the existing Flask app behind FastAPI with WSGI middleware.
• Preserve representative v1 behavior through fallback, including status codes, bodies, headers, CORS, cookies, and country-ID validation.
• Add a native /health endpoint under ASGI.
• Keep production traffic on App Engine/Flask for this PR.
• Avoid schema, database, Supabase, Cloud Run, and native route migration changes.

Additional compatibility work

• Handle budget-window submission-time Modal 4xx validation errors as structured budget-window error results, so staging can tolerate the newer simulation gateway validation behavior while preserving 5xx failures as real errors.

Success criteria

• Existing representative v1 route contracts pass through both Flask direct and FastAPI fallback clients.
• /health returns 200 under ASGI.
• Existing Flask readiness/liveness routes still fall through unchanged.
• No production traffic shift is introduced by this PR.