feat: Support custom ElevenLabs credentials in voice token endpoint

[rrnewton]

Summary

Extends the /v1/voice/token endpoint to accept optional custom ElevenLabs credentials from the client, enabling bring-your-own voice agent support.

Changes

• Custom Credentials Support: Added optional customAgentId and customApiKey parameters to the voice token request
• Credential Selection: Server uses client-provided credentials when present, otherwise falls back to server defaults (ELEVENLABS_API_KEY and ELEVENLABS_AGENT_ID env vars)
• Improved Error Handling: Parse and return detailed error messages from ElevenLabs API responses instead of generic failures

API Changes

POST /v1/voice/token

Request body (all fields optional):

  {
    "revenueCatPublicKey": "string",
    "customAgentId": "string",
    "customApiKey": "string"
  }

When customAgentId and customApiKey are both provided, the server uses these credentials to fetch the conversation token from ElevenLabs. Otherwise, it uses the server's configured defaults.

Backward Compatibility

Fully backward compatible - existing clients that don't send custom credentials continue to work with server defaults.

Test plan

• Request token without custom credentials - uses server defaults
• Request token with custom credentials - uses provided credentials
• Request with invalid custom API key - returns detailed ElevenLabs error
• Request with invalid custom agent ID - returns appropriate error

---

Note: The client and server PRs should be merged together or in sequence (server first, then client) since the client changes depend on the server supporting custom credentials.
Client PR: slopus/happy#258

[rrnewton]

(Update: ran some limited PR tests via the happy-devbox repo that simulates a monorepo linking the commits in these PRs together.)

E2E Tests

This PR includes working E2E tests using Playwright:

• Create Account flow: Tests loading the webapp, creating an account, and verifying logged-in state
• Error handling: Verifies no critical errors during normal operation

CI validation runs ./scripts/validate.sh which includes:

• TypeScript builds for happy-cli, happy-server, and webapp
• Unit tests for happy-cli and happy-server
• E2E tests with browser automation

Green CI Run: https://github.com/rrnewton/happy-devbox/actions/runs/19894554194/job/57021496616