Function handleAnalyze

Handle a POST /api/analyze request end-to-end.

Pipeline:

1. Parse JSON body — return 400 on malformed JSON.
2. Zod-validate the { url } shape — return 400 with a static message on failure (Zod issue detail goes to console.warn, never the client).
3. Download the Drive file — DriveError is mapped to a user-safe response.
4. Analyze with Gemini — GeminiError is mapped to a user-safe response.
5. Return the validated { person, transcript } JSON.

Any unexpected exception is logged at console.error and surfaced as a generic 500. Raw upstream messages never reach the client.