Generate podcast ideas (public, origin-restricted)

Returns 3–8 podcast ideas. Each idea includes a podcast name, description, exactly four episode titles, and AI-estimated 0–100 scores for questionComplexity, topicalDepth, searchVolumeConversational, and contentFreshness. No JWT required, but the request must come from an allowlisted Origin (autocontentapi.com / app.autocontentapi.com / localhost) OR carry a valid X-Free-Tools-Secret header (used by the Next.js proxy). Other callers receive 403. Limits: 2 requests per IP per 24h, 30 total per 24h, 2 concurrent per IP.