Add optional Hey Jude gateway routing

From the PR description

Summary

• Add optional HEY_JUDE_ENABLED routing for Claude, Gemini, and OpenAI model calls.
• Route OpenAI through Hey Jude's /v1/responses compatibility endpoint when enabled.
• Document the minimal Hey Jude environment settings for local setup.

Notes

• Mike still stores original chat text in its database; Hey Jude pseudonymizes prompts before provider calls.
• Full /chat curl testing requires a configured Supabase auth environment and valid user token.

Test Plan

• npm run build --prefix backend
• Live Gemini provider-adapter request through Mike -> Hey Jude -> downstream capture
• Live Anthropic provider-adapter request through Mike -> Hey Jude -> downstream capture
• Live OpenAI complete and streaming Responses requests through Mike -> Hey Jude -> downstream capture
• Capture log checked for raw test entities; downstream payloads did not contain raw Nick Watson or Google