refactor: enhance error handling and streamline API key management in LLM modules

From the PR description

Summary

Hardens workflow listing against shared-workflow crash cases and improves missing API key errors across LLM providers.

Changes

• Wrap workflow route async handlers so rejected promises are passed to Express error handling.
• Add workflow route error middleware to return a controlled 500 instead of allowing unhandled route failures.
• Make shared-workflow sharer-name enrichment non-fatal.
• Replace auth.admin.listUsers({ perPage: 1000 }) with targeted getUserById lookups for only the relevant sharer IDs.
• Keep workflow listing functional even if sharer profile/email lookup fails.
• Add clear missing API key errors for Gemini, OpenAI, and Anthropic before provider calls are attempted.

Why

A shared workflow could cause GET /workflows to throw during sharer enrichment. In Express 4, unhandled async route rejections can escape normal middleware and risk taking down the backend process.

This PR makes the workflow route fail closed and avoids the expensive all-users auth lookup.

Testing

• npm run build --prefix backend