feat(observability): add privacy-bounded endpoint health

[sarthakagrawal927]

Summary

• Add the official App Health 0.4.0 Hono adapter to API and personal-service Worker routes.
• Resolve the existing optional APP_HEALTH_INGEST_KEY lazily; with no key the middleware is a no-op. Keep a bounded queue/batch, one retry, one-second timeout, and background waitUntil flush.
• Let the SDK use Hono's matched static route templates. It sends method, route template, response status, duration, timestamp, and declared response-size scalar; request bodies, headers, cookies, query values, concrete route parameters, account IDs, and profile details are not endpoint telemetry.
• Preserve the existing signup event while replacing its email title and ID/name properties with the fixed title New account created and no user properties.
• Update both source and served privacy-page copies to explain the optional generic signup signal and aggregate endpoint measurements.

The signup-log privacy correction is separate from endpoint metrics but directly related: both use the existing optional App Health ingestion path. The privacy page now describes them as separate signals.

Verification

• pnpm exec biome check src/worker.ts src/worker/endpoint-health.ts src/worker-endpoint-health.test.ts src/server/auth.ts marketing/privacy/index.md
• pnpm typecheck
• pnpm exec vitest run src/worker-endpoint-health.test.ts (4 passed)
• pnpm build
• pnpm exec wrangler deploy --dry-run --outdir /tmp/calorie-app-health-worker-dry-run

No secret was provisioned, no production configuration or deployment was changed, and no live receipt is claimed.

Closes #106