Create an authenticated feasibility interpretation

Creates an AI-backed interpretation for a completed deterministic feasibility analysis, grounded in the frozen analysis facts and persisted for the authenticated user. A Supabase bearer JWT is required for every request; missing or invalid auth returns AUTH_REQUIRED. The route accepts an Idempotency-Key header for safe retries, uses the authenticated-user AI_GENERATE route bucket behind the PUBLIC_API middleware backstop, and returns registered v1 error codes including INVALID_REQUEST, AUTH_REQUIRED, RATE_LIMITED, and ANALYSIS_FAILED.