Add optional Google Sheets submission mirror

[Spbd1]

Motivation

• Provide an optional secondary mirror of completed research submissions to a Google Sheets Apps Script webhook while keeping PostgreSQL as the primary source of truth.
• Ensure the mirror is non-blocking so webhook failures do not prevent participant submissions from returning success once the DB save succeeds.

Description

• Add lib/googleSheetsWebhook.ts with sendSubmissionToGoogleSheets(payload, metadata): Promise<void> that flattens a ValidResearchExport into a concise JSON row, posts to GOOGLE_SHEETS_WEBHOOK_URL with a 5s AbortController timeout, and sets Authorization: Bearer <secret> when GOOGLE_SHEETS_WEBHOOK_SECRET is present.
• The helper omits payloadJson if the full payload is too large, logs concise success or warning messages on failure, and no-ops when GOOGLE_SHEETS_WEBHOOK_URL is unset.
• Call the helper from POST /api/submissions (app/api/submissions/route.ts) after the Prisma researchSubmission.create completes so DB storage remains authoritative and webhook errors do not change the API response.
• Add GOOGLE_SHEETS_WEBHOOK_URL and GOOGLE_SHEETS_WEBHOOK_SECRET to .env.example and add a README section documenting setup steps and a minimal Apps Script receiver that validates the shared secret.

Testing

• Ran npm run typecheck and it completed successfully.
• Ran npm run lint and it passed; output included an existing ParticipantBackgroundForm hook dependency warning (unchanged by this PR).
• Ran npm run build and the production build completed successfully; the same existing hook dependency warning appears but did not block the build.

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Codex Task